I'm pretty sure that if you look on the avengerkink archive, you'll find some smutty Steve/Coulson.

I think the reason it isn't more popular is that the dynamic is kind of goofy, because Coulson is a total fanboy and his fanboying of Steve is a break from his usual demeanor that's played for laughs. However, I have read one utterly heartbreaking one-sided!Coulson/Steve fic that believably treated the pairing totally seriously.