If that's indeed the case, I think it might be because of the fear of license trouble ie the whole business of Docker images referencing/installing a Debian base as the first thing always had the smell of a GPL circumvention device to me, in part at least. But it seems license holders don't really bother, or we've heard about it by now. And in that context, Alpine already got rid of the OS and glib, leaving only the rest of the userland GPLish. OTOH busybox is GPL, too, and actually known for going after violators, so what do I know.