I think it's just gotten to the point where the number of people who are willing to play a very long drawn out turn based multiplayer strategy game (particularly one that does not have simultaenous turns) is a very small niche. When most people want a game that they can play with their friends they usually want one where they don't spend much more time waiting around for their turn then playing. I must admit that while I used to quite happily play some games like that (with strict turn time limits) I no longer really want to either.
People on the other hand love to play cooperative or competitive games where they they can constantly be in the action, which is why there has been such a surge in MP shooters and the like. RTS is also a quite popular field for it. But even if a game like Civ or elemental managed to get the multiplayer working flawlessly, the number of people who actually were willing to play it would be a fairly small demographic compared to the number of people who buy something like modern warfare for the multiplayer. This is why companies don't spend a lot of resources on it.
Civ 4 had a pretty strong MP community. I doubt anybody but Firaxis knows the size of it relative to the SP one, but for a "niche" it was doing pretty well for itself.
Civ 5 didn't manage to reach the same level of support. It's not like people in this market are very demanding in MP: they want SP with some of the AIs replaced by humans. Somehow Firaxis messed that up, with the AI behaving differently in MP and being really dumb (in my last co-op game they were stuck in the renaissance due to building nothing but trading posts on every tile, and the city states were able to beat them). They were completely passive and just braindead. They don't behave the same way in SP. The map generator also doesn't put teammates near each other anymore, which it used to do. That affects competitive and coop play.
I mean hell, you can't even SAVE MANUALLY in Civ 5 MP. This isn't rocket science. It's basic stuff that games have been able to do for decades. Somehow they got it wrong, and in absolute terms MP support in Civ 5 is worse then in Civ 4.
Then you've got Elemental... where they actually put deliberate time and effort into making MP different from SP. Had they spent considerably less time on not doing that and instead just made it possible to have the AI in a MP game, it'd be better then it is. I'm not really sure who is playing competitive Elemental (looking at the Arena it seems to be a small number), but to have a TBS MP game where co-op is flat out impossible? That's pathetic, especially since all we want is the SP game with another human stuck in somewhere.
If you're going to support it at all, you should do it right. When games are coming out today that are worse then ones from four years ago at doing the exact same things? That's bad. That's the point here. FPS games are getting better at it, and TBS games are getting worse.
