Alien races in Trek often express a particularly exaggerated and stereotypical expression of one aspect of the human psyche. From this point of view, the role that the Minbari, Centauri, Narn and Vorlons filled on B5 has, to an extent, already been filled on ST by other races (not necessarily one race, but their particular characteristics are shared amongst several other races).
For example:
Minbari - in B5, we see a race that is somewhat older and wiser than humans, and certainly more aloof. Their society is highly ritualistic and disciplined. Although not identical (they don't suppress their emotions, for a start) they nonetheless fill a very similar role to ST's Vulcans, who are similarly an older, more considered, more disciplined and highly ritualistic society. You'd have trouble integrating the Minbari in to the ST universe without stepping on the dramatic role of the Vulcans.
Centauri - the Centauri are an imperialist culture, complete with political intrigue and backstabbing galore and the complicated power structures that go with it. I feel their role on ST is filled by the Cardassians, who reflect their quintessential property of being 'smooth political operators' and to a certain extent the Romulans, who are convinced of their own self-superiority, and to a smaller extent, the Klingons, for their imperial power structure of great houses and what goes with this. Again, you'd have trouble I think writing a decent Centauri story in the ST universe without simply duplicating these properties of these other races.
Narn - As a warrior race, seemingly prone to occasional uncontrolled bouts of aggression, the obvious counterpart in the ST universe is the Klingons. Narn society include ideas analogous to 'honour' (they seem to be very keen to keep their word), and like the Klingons, they seem to be a very spiritual people but perhaps in a way that other races don't quite 'get'. Their spiritual life is full of great leaders of the past and history passed on from generation to generation, much like Klingons, plus their ceremonies appear to contain a lot of chanting and candles (!).
Vorlons - Now, this is an interesting one. As the supreme 'God-race' of the Galaxy, their role in ST is normally filled by a God-of-the-week. Sometimes it's Q, sometimes it's the Metrons, or the Organians, or Nagilum, or the Cytherians, etc. The Vorlons teach us things about the human condition by showing us how silly and childish we can appear when viewed by an outsider, pushing us to demonstrate our 'human potential' by pushing us in the right direction and showing us that we can exceed ourselves.
B5 made the Vorlons interesting by having one race appear every week, rather than a succession of these kinds of beings. Hence, they might be the easiest and most interesting to integrate with the ST universe. The others would get old and tired very quickly, because most of what you'd want to do with them in a story has already been told by authors writing about races already present in the ST universe.
Just by tuppence, anyway.