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What happened to the Andorians? They were prominent in ENT and even in TOS, but nearly/basically vanished from TNG, DS9 and obviously VOY...

Good question. Budget?

Maybe they turned out to be a bunch of jerks, and the Federation now assigns them all to cargo ships...

Or maybe, global warming melted their planet, and their population decreased dramatically...

Wouldn't that be 'galactic warming'?

As for the Andorians...
IIRC, we've heard of ships with all-Vulcan and all-human crews before. And even the Enterprise only had a handful of non-humans on board. It's possible that there may be a form of speciesism occuring, with various races being stationed on ships crewed almost exclusively by their race.

It's possible. But their absence is weird considering the central role backstory, and later Enterprise, gave them in the early history of the Federation. It has been claimed in non-canon sources that due to the complexity of Andorian mating, they have an extremely low birth rate, so that might account for their relative scarcity.

Outside the universe, I believe that they were never initially developed with as much detail or hsitory as the Vulcans were - anyone ever hear of a guy called Spock? - so the Vulcans became the favorite core Federation non-human species.

That actually makes some sense, in that a mostly Vulcan or Andorian, etc. crew could keep the majority of the ship at a certain comfortable environmental standard. Maybe it would save some power, and certainly it would improve morale/make things a lot less complicated.

Cause in the early 90s most fans would consider Andorians funny looking. Star Trek went more and more complex forehead makeup humanoids, the Andorians are different from all those. I'm honestly surprised they had them in Enterprise, and managed to make them not look like cheap 60s aliens.

OPne of the few things I liked about Enterprise was its treatment of Andorians - and their willingness to portray the downside of the Vulcans.

Indeed. That's one of the few good things that came out of that series.

Well, actually they do appear several times in that era, though most of these 'appearances' consist mainly of offhand mentions. I'm pretty sure this was largely due to the TNG staff's dictum to avoid 'copying' TOS's worlds and species. In the early years, especially, the staff were especially insecure about this kind of thing; and even in later years, they were still reluctant to use any TOS elements if not absolutely necessary.

maybe the next generation, figured that the tellarites and andorians were a little crappy or camp

I seem to recall that when one of the writers wanted to work in a Andorian into an episode of TNG, someone told them, "We don't do antenna". Unsure if this is true or not, nor where I heard this, but the writer created the Bolians to have someone blue in it.

They did once show an Andorian in TNG, as one of the choices of Lal. And it wasn't even blue. Anyway, I rather dislike this, as due to ENT Andorians are now tied with Tellarite for my favorite major Star Trek species. Oddly, another two blue species (Benzites and Bolians) are among my favorite minor ones. I guess I just like blue aliens.

Who's your favorite Grand Admiral then?

Maybe Andorians choose not to join starfleet since it is not military enough for them and instaed have their own defence force.

I would have expected them to show up in the Dominion war...