After studying the history a bit it seems to me the reasons were organizational more than technical. Jobs brought his team back with him and they built on what they were familiar with. With better management the existing nanokernel-based MacOS could probably have been evolved similar to what MS did with NT.
Exactly. MS rehosted win16 apps on top of a new kernel, ported the UI, and encouraged developers to migrate code to win32. Apple had pieces of a similar transition with the transition to PPC but failed to execute, maybe because they were betting on different attempts to rewrite everything from scratch.
I'll admit I haven't seen the NT source but from a user perspective NT's UI (widgets, progman, etc) looked and functioned near identically to Windows 3 so I've always thought those bits were ported over. I'm aware most everything under that was replaced. To my eye that was one step in a well-executed evolution up from legacy Windows towards the eventual convergence in XP.
