Are you referring strictly to the 386 as a UNIX machine?  The 386 shipped in 1986 and the IBM PS/2 Model 80 (with a 386) shipped in '87.  My assumption is that the driving factor in keeping the lower end PS/2 machines around was cost - at launch, a PS/2 with an 8086 was $2300 while one with a 386 was over $10k.  Of course, the 386 PS/2 was initially targeted at OS/2 and not UNIX; from what I can find AIX PS/2 didn't ship until at least late 1988.

-Henry