Was any clear reason given why the other races just let the Centauri republic enslave the Narns either before the events of Babylon 5 and then again when the Centauri attacked again. I know they condemmed the attacks in the council but why didn’t anouther race try to help the Narn?
It's just not that simple.
There's some dreaded p-words driving this answer - politics, and practicality.
- No one was interested in Narn a hundred or more years ago.
By all accounts - and admittedly, this is mostly G'kar's account, but nothing disputes it - Narn was a peaceful and non-spacefaring world when the Centauri originally came in and took over. If they weren't spacefaring, none of the other major powers of the time would have either known about them, or cared to intervene enough on their behalf.
- No one is that interested in helping the Narn now... to start.
Babylon 5 practically starts with G'kar as a hammy villain. He launches small attacks wherever he can, and uses diplomatic muscle to justify them after the fact (Midnight on the Firing Line). When he thinks the Centauri are weak, he uses whatever leverage he can find to create an opening to press more demands on them. He spent most of the first season disturbing the peace in one fashion or another. We get to see another side to him as we watch the show, that he will help even when there's nothing to gain (Mind War) and that he has a spiritual side (By Any Means Necessary), but not all the abmassadors would know that. To them, Narn is expansionist and aggressive regime, and a potential enemy. Some of them may even have seen the Centauri attack as a relief.
G'kar spends most of the Narn-Centauri War getting comeuppance. No one will help Narn now, because they were the troublemakers first.
- Once the Narn homeworld is under attack, it's too late.
Once Narn is being bombarded (The Long Twilight Struggle), the Centauri have pulled in every ship they can spare to enforce their victory. They actually abandoned worlds they were still securing to go all in on this strike. No one power has the military might to challenge the majority of the Centauri fleet all engaged at Narn, unless they fully committed as well. It would probably take several major worlds acting in concert to make it a clean sweep and not risk too much damage to their own fleets... and no one is willing. The Minbari are going isolationist, Earth is spinning towards supporting the Centauri even if B5 doesn't, the Vorlon don't care, and the individual League worlds aren't strong enough to do it on their own.
The truth is, once the Centauri start to worry the others, it's too late to save Narn. The only thing on the ambassador's minds is how to protect their own worlds. Which while short-sighted, is entirely correct, since the Centauri immediately start other wars once Narn has surrendered (Fall of Night).
This was addressed in the episode B5: Acts of Sacrifice.
The Narn are a prideful people and haven't made efforts to reach out to other races to form treaties of mutual defence.
G'Kar: But these outbreaks of violence are jeopardizing our chance to make allies with the humans and all the other aliens.
Narn: We are Narns.
G'Kar: We never needed allies before! We've never been in this position before. We pushed the Centauri off Narn through a war of attrition so that it wasn't worth their effort to stay.
The Minbari are still gunshy after their war with the humans
Delenn: My people are tired of war, G'Kar. You cannot ask them to go through all that again.
and a vaguely contemptuous of getting drawn into what they see as simply being the latest chapter in an ongoing conflict.
Delenn: So do we help you now knowing that in a few years, when your forces are at full strength the Centauri will ask us for help against you? You must know your actions in the recent past the things you've said they make it difficult for anyone to come to your aid now.
Still smarting from their recent war with the Minbari, Earth sees no benefit in getting involved in the conflict. They feel that providing assistance to civilian targets could be viewed as an act of aggression.
We don't have to support their military campaign just to help save their civilians when they get caught in the middle. Captain, I know this isn't what you wanted to hear but we simply can't give the Narn government any kind of official help.
The League of Non-Aligned worlds
Narn expansionism has caused many of the League to form treaties against the Narn. Going up against a resurgent Centauri Republic means that they place themselves in the queue to be attacked next.
G'Kar doesn't even bother asking them.
When the Centauri finally do flatten Narn from orbit it's presented as a fait accompli. The act is already done and although the other powers issue their formal protests, there's very little practical support that can be given since none of the governments are willing to shed blood defending the Narn or enforcing a treaty that Refa correctly identifies as being little more than "words on a page".
ISN: Minbari, Vorlon, Earth and other governments have filed offiicial protests with the Centauri Republic for their use of mass drivers against the civilians. But so far, the protests have had no effect.
Spoiler warning: the question only mentions plot points from season 1. If you haven't watched farther than that, this answer may contain (mild) spoilers for later seasons.
Because the various races have never really historically acted in any way mutually in ways that are for the benefit of the greater community. There's a statement from a later episode (I don't recall which at present), where Delenn says that the thing that distinguishes humans from the rest of the races is that "humans build communities". Therefore the fact that Earth has only recently become a major enough power to get involved in other races' affairs is probably relevant.
And the other point is that to enable this kind of mutual action is exactly the point of why the Babylon project was created: "to prevent another war, by creating a place where humans and aliens can work out their differences peacefully", as the season one opening narration puts it. But as of the start of the show, it hasn't been operational for long enough to have had much effect yet: there's no consensus between the races to act in any way that isn't for the sole benefit of the actor. Making them change this behaviour pattern is clearly an important part of the job that the Babylon 5 crew are going to have to perform over the next few years.
They think someone is helping the Centauri.
Each of them has intelligence services, and they are not fools. Each of them knows that something is going on, a dark shadow is being cast across the galaxy, and they had some guesses as to the sources.
In particular, they were noting things happening in the Narn-Centauri war that weren't explainable by Centauri abilities alone. Perhaps they sent spies to assess battlefields at Quadrant 37 or Gorash 7.
Suffice it to say, they may not be afraid of the Centauri so much as the shadows they see of anther player. This uncertainty gives them pause.
I don't want to spoil the rest of the series if you haven't seen season two and three and beyond yet, but suffice it to say the three big players who might do something, the Humans, Minbari, and Vorlons, have a (much) larger concern. They cannot afford to risk weakening themselves by getting involved in a war that is already effectively over before anyone notices it start. There is a concerted effort to help Narn civilians escape after the fact but militarily the Narn have lost before anyone can do much of anything.