On: Enhanced-mode Windows 3.0 didn't exactly run a copy of standard-mode Windows

Many possible reasons:

- still part of current Windows (unsure about specific code, but 32-bit Win7 still has enough code to run Win3x apps, possible Win8 does but haven't confirmed personally)

- third-party code they don't have full rights to

- trade secrets

- pride

- simply don't care

Your second reason is right on.