Having support from third party vendors is not the same as succeeding because you have the backing of a major vendor (and especially one who created you), like Java or C# or Swift.

C would have been a footnote in the history of programming languages if AT&T didn't offered the code to a few universities, whose students created the UNIX workstations like in Sun's case.

In the late 80's the language had zero value in the hobbyist world.

The few compilers that existed generated crappy code, were not even K&R compatible and only mattered to those that had access to UNIX either at work or university.

C was invented at Bell Labs, a subsidiary of AT&T. Surely that counts as "backing of a major vendor"? C spread along with UNIX, which AT&T as vendor licensed to many other companies. It was also helped along by AT&T's own use of C and UNIX in its internal systems, especially telephone switch development.