Oh I bet fun also played a role. But I don't even know what exactly you are comparing against the PDP-11 here or and what point, because UNIX development happened on 11s for at least a decade. UNIX first ran on a PDP-11 in 1971 or so and a decade earlier there was only CTSS perhaps to compare it to, which had more memory than a PDP-11 at the time. Later timesharing systems like Multics and TOPS-10 also ran on bigger machines. And let's not forget it all began on a PDP-7, which was even smaller.