That success was mostly caused by AT&T not being able to sell UNIX (at least during the first years of UNIX), so they just made the source code available to everyone willing to pay for a symbolic license, which was a tiny fraction of what vendors were charging for their own systems.

An OS and systems programming language available almost for free, including source code, versus what a commercial mainframe, its OS and SDK would cost without source code included, sure recipe for success.