Coming in late here, but DotA's difficulty coupled with its focus on spectating any game, including professional tournaments, provides an amazing viewer experience. It's really one of, if not the best in my opinion, game to watch professionals play. This matched with its large viewerbase means that sponsors are bigger, and tournament prize pots are bigger. MasterCard recently sponsored the recent Alienware Cup tournament [1].

As a side note, Valve has recently crowdsourced the prizepot for The International 3 with great success (their large DotA2 tournament) [2].

[1] https://twitter.com/MasterCard/status/354621397135409152 [2] http://www.dota2.com/international/compendium/