This is a symptom of the approach used for SDLC, Software Development Life Cycle.
When old software is upgraded or new stuff developed, it is done on the latest and greatest OS and registry structures. Few vendors strive to guarantee that nothing will become obsolete.
I won’t say that Big Blue is an exception, but I will say that Big Blue is way, way, way better than most.
One of the big reasons that IBM had to make their systems backward compatible was that their Customers banded together and demanded it. The S/360 was the start of that; most previous hardware changes made a lot of the system programming obsolete, and Blue's Customers realised that they needed a way to help themselves and pressure Blue to "Do the right thing!©" The Users Group SHARE had a big hand in that.