When one looks at GitHub, permissive licences like
BSD-3 are favoured over the weak and strong copyleft licences from the
GNU libraries (for instance, back in 2015, MIT-licensed repositories accounted for twice as many repositories as all
GNU licences combined). Also, there seems to be a trend towards permissive licences against copyleft ones over the past few years.
Taking aside that the popularity of a licence encompasses many different aspects[Note 1] that go beyond the scope of what we can humanly assess here, what are some of the downsides to copyleft licences that would make a developer wary of choosing them for their software?
[Note 1] Two reasons I came up with that are likely to account for their popularity are the ease for a small developer to simply provide a short, simple and direct permissive licence like
MIT instead of having to go through and learn all the lawful/verbose details on
GNU licences, as well as corporations providing the backbone of their frameworks with permissive licences, but then likely developing their own closed-source versions built upon them.
Either way, what I am trying to understand here is not the incentives to choosing a permissive licence, but rather the disincentives to choosing a copyleft one.