I think that this is in large part due to hurdles of paying for software easily.
For example
https://play.google.com/store/apps/details?id=com.iiordanov....
and
https://play.google.com/store/apps/details?id=com.iiordanov....
are the same GPL licensed software either gratis or for $5. Just buying it via the store is straightforward instead of figuring out where the homepage is, finding a donate button, entering credit card details... .
Having more non-gratis but free software would be a good thing