Hypen, the answer might lie in the fact that with online betting, people don't get their winnings in their pocket. It just goes into their account. People vary rarely cash in their account. By paying out earlier, you are effectively giving gamblers more opportunity to re-gamble that money, and eventually the house always wins.
If they don't pay out, and the (likely) winning bet sits there for another two months unrealised it's effectively a liability sitting on the agency's books.
And for the promotional aspects, as pointed out above.