I have no idea if there was a conscious choice either way.  However the goal was certainly, &quot;Our cooperative multi-tasking system where any bad application takes out the operating system no longer cuts it, migrate to a preemptive one.&quot;  And building on Unix was the shortest path there.<p>There were many disconnects between the two systems.  See <a href="https:&#x2F;&#x2F;www.usenix.org&#x2F;legacy&#x2F;event&#x2F;usenix2000&#x2F;invitedtalks&#x2F;sanchez_html&#x2F;" rel="nofollow">https:&#x2F;&#x2F;www.usenix.org&#x2F;legacy&#x2F;event&#x2F;usenix2000&#x2F;invitedtalks&#x2F;...</a> for some of them.