NeXTStep was built around BSD, and so BSD followed through to MacOS, which is really frankly just re-skinned NeXTStep. I'm pretty sure it would have been a real PITA to rework it around Linux at that point. Lots of existing applications and tools would have had to have been reworked.

And at that time (late 90s) the Linux kernel was certainly awesome but not clearly that much more awesome than BSD. The amount of driver and architecture support in the kernel wasn't as expansive as now.