The delay in publishing a "proper" standard was due to the incredible success/usefulness of the defacto K&R standard. But as you point out that was hard to find outside of Unix. I suspect this was mostly due to the effort required to implement the full standard library and/or resource limitations on many systems.
For example, there was a Small-C compiler available for the Atari 800 in 1982:
"... based on the Small C compiler published in Dr. Dobb's Journal"
If you look in the beginning of the manual it has a list of what is and is not supported. They claim it is sufficient to compile C/65 itself but there are lots of things we take for granted missing.
So it is kind of ironic this revisionism how great was C "portability", when in reality it was full of dialects outside UNIX just like the competition.
For example, there was a Small-C compiler available for the Atari 800 in 1982:
http://www.atarimania.com/utility-atari-400-800-xl-xe-c-65_1...
"... based on the Small C compiler published in Dr. Dobb's Journal"
If you look in the beginning of the manual it has a list of what is and is not supported. They claim it is sufficient to compile C/65 itself but there are lots of things we take for granted missing.