I always just assumed it was mostly because of the desire to have a proprietary system he could generate revenue out of, instead of wading into GPL-esque territory that tends to go along with LISP.
It's also one of the main reasons I have been using Octave and Python/Anaconda.
One thing I'm sure was very significant was an IP fight with his institution, Caltech, which prompted him resigning in 1983, the same year RMS gave up on Lisp Machines and started the GNU project.
Allegro Common Lisp, Franz Lisp provide IDE tooling reminiscent of the Lisp Machines. With visual debuggers, graphical REPL, hypertext documentation triggered from the REPL, libraries to integrate with the OS....
Additionally their optimising compilers are quite good.
It's also one of the main reasons I have been using Octave and Python/Anaconda.