There are so many exploits developed every year. How can it matter very much if I disclose mine? Someone else will, I'm sure. They're all running the same JS fuzzer farms, after all. Who cares if Latvian organized crime gangs find it first? Someone good will find it eventually. Or at least reverse it out of the rootkits.
The problem isn't simply that the procedure is different. It's that the procedure is clearly, obviously, demonstrably sub-optimal. The Chrome bug-bounty, which Zalewski is surely involved with, runs year-round. The incentive is, you find something, you file it with them ASAP before someone else does. The same is not the case with CanSec's Pwn2Own, where, for no reason other than marketing for ZDI and CanSec, disclosure happens at one precise time during the year.