Another part was pretty vicious infighting between PDP-10 (already sorta on thin ice from start due to fuckup that was PDP-6) and PDP-11/VAX teams, and VAX team won it pretty decisively - if in part through consolidation efforts.

EDIT: Another major difference was how PDP-10 was from an era where everyone with a (under)graduate students to throw at the problem was making custom modifications to hardware.

Like some early multiprocessor PDP-10 being done with hand-soldered MP bus done by students.

It was a wild place