Dev tools.
Remember Steve Ballmer's "Developers! Developers! Developers!" dance? https://youtu.be/I14b-C67EXY?t=10
He was right. Without developers for a new OS, you are dead in the water. Which is my MS has fought so hard to keep compatibility.
Apple had to switch OS. That meant it had to persuade all its devs to switch OS. That meant it had to offer the devs something very special, and that something was NeXTstep and Interface Builder. NeXT's dev tools were the best in the software business and _that_ offered trad Mac devs a good enough reason to come across.
Be had nothing like that.
BeOS was wonderful, but it was not a replacement for NeXTstep as a replacement for MacOS.
But there was another company out there.
BeOS was a natively multiprocessor OS, when that was very rare. One of the reasons is that in fast x86 computers, the x86 chip is one of the most expensive single components in the machine, and it puts out most of the heat.
Especially at the end of the 1990s and early 2000s, the era of big fat Slot 2 Pentium IIIs and worse still Pentium 4s.
But there was one company making powerful desktops with the cheapest, coolest-running CPUs in the world, where making a dual-processor machine _in 1998_ was barely more expensive than making a uniprocessor one.
That company's CPUs are the best-selling CPUs ever designed and outsell all x86 chips put together (Intel + AMD + Via etc.) by well over 10 to 1.
And it needed a lightweight, SMP-capable OS very badly, right at the time Be was porting BeOS from PowerPC to x86...