A majority (if not most) of the contributions to the Linux kernel come from developers paid to do so by the likes of Intel, Google, IBM etc. [1]. Do you have any reason to believe that said for-profit companies would have contributed to the same degree to a BSD/MIT licensed Linux kernel instead of focusing on a Linux kernel with proprietary extensions under a closed license?
[1] http://www.linuxfoundation.org/news-media/infographics/who-w...