There are plenty of successful open source projects with permissive licenses. GPL was probably the best license for Linux at a time when the debate was rife with a"winner take all" attitude. Today, maybe a permissive license would have sufficed.
A permissive license would allow Android phone manufacturers to legally keep their kernel closed-source. Not so good for custom ROM developers.
Even with a GPL_ed kernel, you need binary blobs for certain firmware and Android drivers. The base kernel is only half the story.