You would realize that it wasn't the Different systems that required different versions of Unix because they where different makers but because they where completely different systems with different means of doing the same thing.
Unix itself wasn't so different on each different platform unless you included what the different hardware required to work. Hence you could not take a program developed for one Model of Wang and use on a IBM or even in most cases a different model of the same makers systems.
You can hardly claim that the different systems where so radically different by just looking at the form of th OS that they used. Back then even in the unlikely event of Bill and Co introducing Software for the Main Frame Market you would have found exactly the same situation different versions of the same software is required for different Platform Makers systems and different systems within the one make.
It's just the same as making one driver set and expecting it to work on all Hardware Platforms where you are not controlling the actual Hardware like Apple does.
Even today M$ doesn't work of different hardware unless you install the necessary Specialty Drivers for that Hardware. Back then it wasn't so much a matter of different drivers as completely different hardware which worked in different ways.
If you want to believe that the PC has Standardized Computers you should be thanking IBM not M$ as it was IBM who made the PC and they just went to M$ for software which I should add M$ didn't have back then. They just wrote software and in no way where involved in OS's in the very beginning. They only got involved in OS's because of the way that IBM went about their business practices as even at the very beginning of the IBM PC M$ didn't want to be involved in DOS they never wrote a single line of code for it and actually recommended to IBM that they speak to Digital Research for a OS. I know that and I wasn't involved with PC's back then.

Col