I don't know if this is true. In the 80s there were many languages that were C with additional features or C preprocessors that added and experimented with features similar to cfront. You had OOPC (object-oriented pre-compiler), Objective-C, C*, Concurrent-C. People were experimenting in all kinds of ways by taking C and trying things out with it.
I think it is absolutely true, because adding features to an experimental language that has no tools or ecosystem surrounding it does nothing and people know that.
Niche experiments having features doesn't accomplish anything, but adding just one more feature to C seems plausible.
With C++ people could point people to another production ready language compatible with C that people could use, so there was somewhere they could do and an example of the feature working instead of someone promising silver bullets in theory.
> adding just one more feature to C seems plausible
I thought it would be a couple months to add C++ to my C compiler. 10 years later...