I think market share has mostly led to that. As long as the commercial unixes had significant market share (esp. among paying customers), people developing Unix

I think market share has mostly led to that. As long as the commercial unixes had significant market share (esp. among paying customers), people developing Unix software had an incentive to write it portably, so it could run on at least Solaris and Linux. As the bottom dropped out of the Solaris market, a lot of packages decided not to bother, as >90% of Unix customers were on Linux. Some don't even bother supporting more than one specific Linux distribution (among commercial software packages, that's often RHEL, sometimes RHEL+Ubuntu). There was a period when non-x86 compatibility went through a similar low period (due to the decline of Alpha, SPARC, etc.), but increasing ARM market share is making architecture portability more practically relevant again.