As mentioned, it's simply not possible to run 16 bit software on a 64 bit system. Microsoft COULD have made a Windows On Windows setup that would do it - all the components are already there (they can run 16 bit software on a 32 bit system, and 32 bit software on a 64 bit system, ergo they COULD run 16 bit software on a 64 bit system).
But not having provided that, they incentivized software authors to upgrade their applications to at least 32 bit which, coincidentally, is where most software stopped at.
The interesting thing is that while binary compatibility is provided by their WOW implementation, it's really quite trivial for a software writer to recompile his 32 bit program to run on a 64 bit system - yet many have never taken that step. I'm sure they think, "why bother?"
I wonder when (or if) Microsoft will obsolete 32 bit software (i.e., remove the WOW system entirely from their 64 bit systems). I'm kind of surprised they didn't test those waters with Win 8 or 10. They're all about deleting things to make the system easier for Microsoft to maintain, and I've got to believe that the WOW system is complex.