Notch wasn't the only owner. In fact, he owns less than 50% and they have 40+ employees.
http://en.wikipedia.org/wiki/Mojang
Open sourcing it would likely screw his co-founders, employees with stock options, etc. compared to the size of this deal.
Tbh, I'd be happier if he donated 10% of his earnings from this sale to Open Source projects and feeding people. Rather than, y'know, having gone the route you suggested. He also seems like the kind of guy that would.