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.
To be fair to them, they wanted to create a language whose compiler did fit into the limited hardware they had.
It's not their fault that UNIX had so much success and the world largely went there ignoring most of the progress made in programming tools and languages during the 70s.