My question is why this tool is not used/mentioned more than it is?
It's probably because of history, and because how we humans generally behave.
Traditionally gcc has been the only real (free) compiler that can be practically used to compile C programs on atleast all the free *nix clones out there. It's what virtually all the base system and kernel of linux, *BSD, now probably OSX, and others are compiled with.
While flaws are here and there, basically this means: gcc works. And if it isn't broken, don't fix it. Out of this, you now have a huge user base, it's easy to get help with gcc, there's a lot of people that have used gcc, that are working on gcc itself etc.
Generally, if you want to switch a huge community from something they're used to, to something else, that "something else" have to be *significantly" better. Just "better" is often not reason enough. I think you can find examples of this in many areas of society.
clang is newer, some people will just be suspicious if it's up to the task, if it has bugs, if it produces slower code etc. - it seems to be in the human nature to be suspicious - new things are scary. Many don't even know about clang, many don't care because they're happy with gcc.
Though, if you rather want to use clang, go for it - error messages are indeed "better" and easier to understand vs gcc.
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