I think whenever you have a multitude of planes in a given mission people will gravitate in two directions:
To the planes they're most familiar with/best at
To the planes that are most effective
To balance a mission in such a way that the guy in the Su-25 (non-T) has stuff to do before the 50's and 10's blow it all up isn't easy. Unless you make ALL ground pounders Su-25s to force the issue, that is. But if you can kill 10 targets in 10 mins in an A-10C or only one in an Su-25, why take the Su-25 and get 2 kills while the A-10 takes out the rest of the attacking force alone? You feel useless. If you feel penalized when flying anything other than an A-10, why would you?
I don't know why the Russian jets in FC3 aren't that popular but I have two complementary theories there:
1. Western pilots fly on Western servers and prefer Western jets. The Ka-50 is flown only because there's no precision attack Western helo, but if there was an AH-1W or AH-64A the 50 would never be flown. The Russian jets get more attention on non-Western servers where English-speaking pilots aren't visiting.
2. Fatigue. Aside from some 3D model changes, flying the MiG-29 or Su-33 in FC3 is fundamentally the same as flying it in LOMAC, and how many hours have most of us already put into those planes? The F-15C has received some updates, plus it's the only Western AA fighter in DCS right now, so naturally it gets the lion share.
Personally I'm not in camp #1, I like the Russian birds just as much. In Il-2 I flew US, Brit, Japanese, German, Italian, I didn't care. I do find the stock 25 hard to be very effective in, though, so while it's ok in SP when I'm flying coop online I tend to ignore it in favor of a plane I can get more done with in less time so my wingmates don't take out the entire enemy army while I'm setting up pass #2!
But I am in #2. After hundreds of hours in the Su-27 going back to Flanker 1, I don't care to fly it too much right now because it's the same experience, just in slightly different terrain. I don't even fly the F-15C too much because it's still fairly the same and the missile and FM can irritate me.
So I fly the 25T, A-10A/C, and Ka-50 most of the time!

The Jedi Master