DotA has a lot of prizes of money simply because it's sponsored by SMM. SMM is a Malaysian company that supply Blizzard games in Malaysia and have affiliated with Blizzard, so they don't sponsored other game that developed and published by Blizzard. With that affiliation, of course they have a lot of money from themselves and additional money form Blizzard to support any tournament that only has Blizzard name behind the game. It is simply because of that.

DotA more popularity because more people playing it, and to remind, it is quite an old game. COD4 not even 1 year age in Malaysia, now the population of COD4 is increasing. If there's a lot of COD4 gamers in Malaysia, any sponsor would interested in sponsoring any tournament of COD4.