Why did GNU choose UNIX?

Or rather, why did rms et al dislike UNIX? It seems to come up a lot that they basically settled for a UNIX clone since it was widespread. They certainly didn't care for the philosophy.

One major factor in the selection of Unix was its portability. At the time, RMS was already mourning the passing of the ITS system. All of the core software that composed ITS was written in PDP-10 assembly (as were most operating systems of its type), so when the PDP-10 died ITS died with it. The same thing would happen to MIT's Lisp machine culture when Symbolics ceased waking life.

At the time, Unix was the only OS prevalent in academia that had been widely ported to multiple computer systems. RMS concluded that if he could use Unix as a base for his system, the wider Unix community would assure its continued survival. This is exactly what happened when the Hurd effort failed - the GNU project simply adopted another Unix-like kernel in its place.

Incidentally, this approach explains why many of the GNU tools have this weird split personality. Things like info or mandatory long options are near useless to people who are accustomed to Unix shells - their real purpose is to be used as primitives in an environment that bears more semblance to a Lisp machine (namely, emacs).