Windows kernel exploits are worth more because they're worth more on the open market (because that's where the corporate data is and corporations pay ransoms). pwn2own has to compete with the black market, after all. If you discover have a Windows exploit - you can sell it for a lot of money if you sell it exclusively. Not so much an OSX and even less a Linux desktop exploit. So market forces dictate that, if you want people to actually turn up to pwn2own and show you their exploits, you need to make it attractive, not just to pure whitehats but to greyhats, too. If they can get $50,000 or something from "some guy in Russia" you can't very well offer $5,000 and hope they tell you out of the goodness of their hearts.