At the time Apple was a bit more open to such experiments than it is now. There were the licensed clones, there was the Macintosh Application Environment allowing to run Macintosh applications on Solaris and HP-UX, and Apple was indeed in need of a new, more modern OS.
So it wasn’t completely crazy to go with that. It also provided a much less diverse choice of hardware, at a time where Be could not have done a generic OS supporting the dozens of different video and network cards used on PC (even later on, BeOS was known for not-so-great hardware support, until OpenBeOS/Haiku developers started providing drivers).
Of course, looking at it from where we are now and knowing what happened next, it’s easy to say it was the bad decision. But at the time, it certainly wasn’t so obvious.