I am of course guessing here. It's possible that I'm completely wrong since I've never been in the working group and don't really know anyone on the working gro

I am of course guessing here. It's possible that I'm completely wrong since I've never been in the working group and don't really know anyone on the working group. But given what I've read publicly this seems like a likely explanation: Because not supporting Get would break a lot of current web applications and html5 had a pretty clear goal of being mostly backwards compatible. The previous form element didn't restrict the method and GET and POST are used everywhere. Restricting GET would have caused too much confusion to be worth it.