I think it's mostly that C's lack of language features removes distractions. You have only a couple abstractions to work with if you're writing idiomatic C code

I think it's mostly that C's lack of language features removes distractions. You have only a couple abstractions to work with if you're writing idiomatic C code, and so you can focus on the task at hand more easily. The tradeoff is that the task at hand tends to have more implementation details in it than in higher level languages, but as you said, this isn't always bad.