People do play the 3v3 map, competitively. It still isn't as popular as the typical moba style map that is Summoner's Rift. My point in saying, "this isn't an FPS," is because FPS game don't have all the champions and such to balance as much as a moba would, so they can pump out new maps.
The more maps they have the more different jungles and such they might have and I think it'd be difficult to keep the balance in check. For example there are a lot of champions you would probably never see in Dominion because they're balanced around Summoner's Rift. There are two ranked maps, all the pros prefer Summoner's Rift over the other maps because I believe that's what the game is essentially designed around.
I'm down for them having similar maps to Summoner's Rift because that gives the essential moba feel of the game that Dota, Dota 2, HoN, and LoL have on their standard map. They could change the jungle positions up, maybe have different buff monsters and dragon/baron type objectives. So instead of having some map where you capture points, which is nothing like Summoner's Rift, so not every champion is viable for the map, they would have one that feels like Summoner's Rift, but it would have different strategies. I don't think 3v3s are as popular because its 3v3 and pro team can't easily go from 5v5s to 3v3s without omitting some of their members.