The success of FOSS can in large part be attributed to copyleft licenses like the GPL. Without the protections of copyleft clauses, software just gets exploited by large corporations and end users are locked out. For just one example, if GNU software had used MIT, the entire free router movement (i.e ddwrt, openwrt and co.) would probably not exist today.

See: Free Software Foundation, Inc. v. Cisco Systems, Inc..

Edit: actually, I think by the time of this specific lawsuit, the sources for wrt54g were already released after community pressure, this article details the history a bit better.