a) Every new sale gets them $200, much more for the bells-and-whistles financial version. It won't take many sales to make back the prize money.

b) It's being used largely inside organizations, which need a push to share what they're doing with resolver. [I'm in this situation: I've done some cool things with resolver, but been asked not to talk about them. I may use the prospect of prize money to nudge my client into loosening up on the secrecy]

c) It's a very good piece of software. People trying out the app are likely to come away loving it.

d) Potential customers may be worried about buying something with a small user-base. A competition like this makes the number of existing users more visible, and maybe draws in new ones.