[herewulf]

Point taken but at that time there was no other free (as in beer and freedom) "UNIX" kernel?

Someone may come along and correct me about BSD. Apologies I'm not super familiar with it's history.

[Joker_vD]

There was GNU Hurd in development at the time (which is actually a microkernel), with the first public release in 1990. Needless to say, it never amounted to much.