ERerichocean·Oct 10, 2013
How about the usual reason? Because it can't be pulled into non-GPL projects.
This is actually relevant, at least, to me. I've been porting Twitter's fatcache[1] to iOS, and wouldn't be if it was GPL'd. I haven't even looked at Facebook's code, due to the GPL license, which makes it—literally—unusable to me on iOS and Android.
[1] https://github.com/twitter/fatcache
In FlashCache at Facebook from 2010 to 2013 and beyond