Being easily affordable/available in those times was the initial "hook" but C's subsequent and sustained success was due to a happy confluence of various design decisions.

Not too high-level, Not too low-level, easy access to memory/ISA, simple abstract machine, being imperative procedural, spanning bare-metal/OS/app, adopted by free software movement producing free compilers/tools, becoming de-facto industry standard ABI etc. all were crucial in its rise to power.

Note that its main competitor at that time, Pascal; lost out in spite of being simpler, having clean high-level features, promoted by academia, safety focused etc.

As Dennis Ritchie himself said in "The Development of the C Language" (https://www.nokia.com/bell-labs/about/dennis-m-ritchie/chist...);

C is quirky, flawed, and an enormous success. While accidents of history surely helped, it evidently satisfied a need for a system implementation language efficient enough to displace assembly language, yet sufficiently abstract and fluent to describe algorithms and interactions in a wide variety of environments.