I think C's success is also because it was designed while solving real problems when writing UNIX. Unfortunately the newer languages that claim to be "systems" languages were not designed while building operating systems. Here is Dennis Ritchie's assessment of reasons for C's popularity:
(Extract from http://csapp.cs.cmu.edu/3e/docs/chistory.html).
C has become successful to an extent far surpassing any early expectations. What qualities contributed to its widespread use?
Doubtless the success of Unix itself was the most important factor; it made the language available to hundreds of thousands of people. Conversely, of course, Unix's use of C and its consequent portability to a wide variety of machines was important in the system's success. But the language's invasion of other environments suggests more fundamental merits.
Despite some aspects mysterious to the beginner and occasionally even to the adept, C remains a simple and small language, translatable with simple and small compilers. Its types and operations are well-grounded in those provided by real machines, and for people used to how computers work, learning the idioms for generating time- and space-efficient programs is not difficult. At the same time the language is sufficiently abstracted from machine details that program portability can be achieved.
Equally important, C and its central library support always remained in touch with a real environment. It was not designed in isolation to prove a point, or to serve as an example, but as a tool to write programs that did useful things; it was always meant to interact with a larger operating system, and was regarded as a tool to build larger tools. A parsimonious, pragmatic approach influenced the things that went into C: it covers the essential needs of many programmers, but does not try to supply too much.
Finally, despite the changes that it has undergone since its first published description, which was admittedly informal and incomplete, the actual C language as seen by millions of users using many different compilers has remained remarkably stable and unified compared to those of similarly widespread currency, for example Pascal and Fortran. There are differing dialects of C—most noticeably, those described by the older K&R and the newer Standard C—but on the whole, C has remained freer of proprietary extensions than other languages. Perhaps the most significant extensions are the `far' and `near' pointer qualifications intended to deal with peculiarities of some Intel processors. Although C was not originally designed with portability as a prime goal, it succeeded in expressing programs, even including operating systems, on machines ranging from the smallest personal computers through the mightiest supercomputers.
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.