There's a new map coming called Magma Chamber that is a 5v5 but is bigger than Summoner's Rift.

Maps take time to develop due to balancing issues. If you've played on both Twisted Treeline and Summoner's Rift you'll know why. The metagame and choice champions are very different on both. TT depends more on tanky and strong early game champs, while SR allows for a variety--though if you focus too hard on say, late game champs, you might lose completely early game before you even get there.

In addition to what psyche said, you want the maps to be different enough to justify having multiple maps, but this will splinter the player base increasing queue times and hindering matchmaking's attempts to make relatively fair matches. I think one new map a year isn't a bad pace, as it lets you learn about the current maps without overwhelming you with more information than there already is.