well, because it's awesome?? no seriously if you enjoyed Defense of the Ancients then you will enjoy dota 2 even more...
it has a quite good matchmaking system, tons of heroes, there is also a real attention to hero balance and there's no real OP heroes, there's also a spectator feature, and Icefrog the map editor from W3 is still in charge of balance and hero creation
it still has the ridiculously steep learning curve but once you get use to it, you will enjoy it tremendously..
