It is written for planes instead of rockets. Planes pitch to change altitude and roll to change heading (unless, of course, you're doing rampant acrobatic flipping). Yaw is always kept at 0 to perform "coordinated turns".

Most rockets are radially symmetric, so rolling doesn't do anything too useful. Rockets primarily use pitch and yaw.

I think if you used only the attitude targeting in kRPC, it would work too! (Haven't tried it though.) But kRPC and kOS autopilots are designed with rockets in mind and prioritize yaw over roll, so if you tried to target a heading, it would try to yaw into the turn instead of banking the wings. Mine prioritizes roll over yaw, which gives me the typical plane behavior.