Centauri have telepaths, so
they have been manipulated by Vorlons like all other races with telepaths. The angelic appearance of Vorlons is also a result of this manipulation.
When Londo asked what he saw when he looked at Kosh outside his encounter suit, he replied "absolutely nothing". How come?
There are a couple of ideas in the analysis of the episode in the Lurker's Guide to B5:
Londo failed to see Kosh when he revealed himself. Does this extend to all Centauri, or is it peculiar to Londo? If it is the former then it suggests that either the Vorlons have not openly visited the Centauri (why?) or that their worship of their deceased Emperors as gods has diminished the effect of exposure to Vorlons. If it is the latter then it must be because of Londo's association with the Shadows. If this is the case then what would be the response of other Centauri on seeing a Vorlon?
And some comments from J Michael Straczynski, the writer, on the same page:
Nnnnnnoo, not really; the Centauri don't actually have an equivalent to G'Quan or Valen.
They [Centauri] believe in a variety of afterlives; the god you worship, of the centauri pantheon, holds dominion over a given "heaven" or afterworld. If you appease the god sufficiently during life, it will accept you into that afterworld, in preparation for the day when all heavens are united; if not, you will have to be reborn and choose another until one accepts you.
Maybe the answer is simpler than people are trying to make it. Perhaps Londo Mollari didn't see anything because the Centauri Emperor was right: He's damned.
Everyone else saw a vision of hope in Kosh. The damned rarely see hope in the light; they must blind themselves to it in order to embrace their own damnation.
Just a thought, but Kosh is the one who projects himself to everyone, that is why it was so exhausting for him to do it, the concentration needed to show a different "deity" to everyone depending on their beliefs. Given that it is a conscious decision by Kosh to project himself and that Londo has already been influenced by the Shadows, it seems likely that Kosh decided it wasn't important to project anything to Londo, as it would have just drained him that much more for no real purpose.
Though it isn't stated in the series, it is implied that other Centauri do see something when they look at Kosh, given the entire stations reaction to the "miracle", so it is likely that Kosh just decided not to project to Londo.
With Londo being the go to person for the Shadows, they may have adjusted him psychically so that he would be immune to the Vorlon's "Majesty". By the time of Kosh's reveal he had been in the presence of Shadows multiple times, possibly for prolonged periods. And the Shadows had invested much in Londo, so some protection from him "seeing the light" would probably be in order. If at that time the Centauri were shifted away from the Shadows it could have been a costly delay. They could have developed other tools among the other races (Drazi spring to mind) or bypass Londo for Reefa as they did later, but that would require time, their most precious resource.
I always saw it as reflecting Londo's cynical spiritual state. Everyone else saw reflections of a higher power they knew as being "good" (ie, angels or deific figures). Londo, even though he participates at various times in Centauri festivals and such with religious themes, really I think does not believe in them. He is, in a word, "fallen," and unable to see transcendent "good."
This is actually touched on, ever so briefly, in one of the tie-in novels.
This is from Out of the Darkness, which is set during the time Londo is Emperor and the Drakh are pulling the strings on the planet. During a meeting with his advisers, he's trying to relay Kosh's comment on avalanches and pebbles (surprisingly, for not having been the one to hear it, he gets the meaning right, if not the words, he goes with blizzards and snowflakes), he reflects on it...
... but I was present that amazing day when Kosh Naranek, the Vorlon ambassador, emerged from his suit. Others reported visions of a great winged being, and I...
I saw nothing.
Actually, that is not entirely accurate. I saw... light. An overwhelming brilliance. But it was shapeless, amorphous, and indistinct. For a moment, it seemed as though I perceived a hint of something, but that was all.
Sometimes I have wondered whether what others saw was some sort of mass delusion... or whether I was simply not deserving of the experience.
This is only Londo's reflection on the moment, but it may be right from both angles. We know for a fact that Kosh projected that perception of a winged angel to all those present, so it was a mass delusion, just not an accidental one - one that Kosh was responsible for.
By implication, if Londo is also right on the other... this means Kosh didn't bother to include him. This could be due to Londo's growing darkness, his association with the Shadows, conserving his own energy, all of the above, or any other reasons that would only be known to the enigmatic Vorlon.
(I think other answers have covered pretty well, between the possibility of other Centauri seeing Kosh and the fact that telepaths showed up there just like everywhere else, that it was not a matter of race)
As well as the excellent answers by Daniel Roseman and NominSim, I'd always understood it to be that all the other species reacted by seeing Kosh as an angel because the Vorlons had manipulated all of us to react that way.
The Centauri, however, had been satellites of the Shadows (see "Movements of Fire and Shadow", 5x18), so the Vorlons wouldn't have had the opportunity to manipulate them thus.
I think all these answers miss an important point: Londo merely claimed he saw absolutely nothing. My interpretation was that what he saw made him uncomfortable and that he did not wish to discuss it. Giving that answer effectively ended conversation on the topic. Londo was always a master of manipulating conversations.
Of course, none of this speculation precludes the idea that he was telling the truth.
Londo sees nothing because he is "lost" in a moral sense. He's given up all of his ideals, and now believes that there is no good or bad in the world. He believes the things he has done are unforgiveable, and all he has left is his selfishness.
It's not a religion thing: Sheridan sees a generic figure of light, presumably because he doesn't have a religion.
So presumably, to see a Vorlon you have to believe (at least a little) in the things they stand for. Things like cooperation, altruism and preservation of life.
The Vorlons and the Shadows were antcient mortal enemies. This is the basis of the war, a proxy war. The Vorlons and their telepaths on one side and the Shadows and their minions on the other. Kosh would not reveal himself to Londo because he was in league with, or as he put it "has been touched by the Shadows".
A being of Light ought to be perceived by any other living being. For all living beings are partly of Light.
Were Londo not partly of Light, he would not have regrets in his life, or for another example, he would not have watched the attack on the Narn homeworld with discomfort.
When he stated that he saw nothing, two possibilities can explain his answer.
One, having been "touched by the Shadows", having immersed himself in Darkness, he felt uncomfortable not only in the presence of Kosh but also at admitting he witnessed a gesture of sacrifice - a point which was presented in exactly the previous episode with the Inquisitor. Sacrifice not for millions only but even for one single soul! Even if this sacrifice remains without acknowledgement. Spoiler yet another similar display of the thesis, when later on Z'Ha'Dum Kosh tells Sheridan to let (his life) go.
Sacrifice, those "touched by the Darkness", those in the Dark Side cannot afford.
Another alternative is if we assume that
Londo has no soul.
Being influenced by the shadows. What if he saw the equivalent of the devil? Which explains his hesitation to say what he saw and the look he gave when he saw Kosh. Kosh being Kosh he also knew this would lead the Centauri to ally with the shadows, which would be how history is supposed to play out.
There was a Hidden agenda between the Shadows and Vorlons and as we know they did not break rules so maybe they knew Londo belonged to the Shadows and they could not touch or even did not want to know, why waste energy projecting to anyone unimportant as we know it did take many days for Kosh to recover.
