How is the lack of Suliban accounted for in the timeline after Star Trek: Enterprise? I have to ask because it didn't make sense to me that a new species would be introduced in a prequel without an explanation as to why they are absent in the future timeline of the Star Trek Universe. I referenced Detained episode of Star Trek: Enterprise, but wasn't able to resolve the question.
The focus of Star Trek, in all incarnations, was to show a diversity of life in the galaxy. In the Star Trek: The Next Generation series bible, Roddenberry states that the point is not to keep telling stories about the characters or descendants of the characters in the original show. He also made it clear that they were to tell stories about new aliens, not to focus on the same ones over and over.
His point was that there was a huge diversity of life in the galaxy and they didn't need to keep revisiting the same planets over and over. Also, there's no need to visit a planet where everything is going well.
Also, the Suliban had lost their homeworld and scattered to different planets. At first they were a problem, but even in Enterprise, after a while, we saw less and less of them.
This indicates that, as time passed, they either settled down and became less of a problem, or were more concerned with survival than interfering with other species, or they were dying out.
Putting these together, and taking into account that the Suliban became less noticeable in later seasons (and less interested in humans), even in the show Enterprise, interactions with the race was waning.
That leaves four possibilities:
1) They died out, probably due to the scattering
2) They settled down and worked on rebuilding their society
3) They were so fractured, they had to focus on just surviving in all their colonies
4) The Suliban that were seen by humans were the ones left over from the "scattering" of the race that didn't want to settle down in a colony and eventually they all either settled down or died out.
This is not addressed in canon anywhere or in Memory Alpha or anywhere else I could find, so I can't give you an authoritative answer, just the possibilities and likelihoods (and from what I could find, I consider #4 most likely).
The simplest way to "account" for it "in show" is to simply say "It's a big galaxy, and we never ran into them". Nothing more complicated required.
Just re-watched Storm Front. According to Silik, Vosk's faction went back in the Sulibans' history and prevented them from achieving sentience. Some Suliban, mostly those working with Silik, were spared the effects by Crewman Daniels's (Starfleet?) faction. But I doubt that would be a large enough gene pool to ensure the species' survival. Moreover, the preserved Suliban in question were genetically enhanced, which could have done gods-know-what to their ability to naturally reproduce.
Star Trek Online answers this question. Not sure if it's counted as canon, but regardless, there are Suliban in Romulan space, relatively isolated from others. Since we rarely go over the neutral zone in the TV series and films, it makes sense that we wouldn't have seen them if they'd stayed there since Enterprise.