In all honesty the K&R book is only an introduction and and a minimal reference to the language. Very good at achieving those goals though. It fails pretty hard at explaining how to use the language and what are the pitfalls of it when applied to application development and why it doesn't hold your hand like many higher level languages do to protect your from making silly mistakes.
So I guess it comes down to the fact that people use what they know, what they are comfortable with, right tool for the job, etc.... I would imagine it's the same issue why we don't see LOGO, ADA, or Forth in common use (I know that Forth is used as the FreeBSD 3rd stage bootloader when booting the system). Fortran and COBOL are still used in their respective arenas (scientific and business apps), but that is going by the wayside as well. Although Java is still a relatively strong contender, most other non-web languages are becoming victims of the C++/Python bandwagon.
I have a copy of the K&R book myself...but I didn't learn C from that one. I initially took a class in C. I had issues with it. Then in the late 1990s, I started to use FreeBSD so I had to learn C. I did so from a different book. I have been using it ever since.