2. She Wasn't That Clever
Hear me out on this one, okay? No one can be smarter than the Doctor. No one can equal his knowledge and experience. The closest we came was River, and talk about a divided fanbase! So companions are all varying degrees of what, compared to the Doctor, is "stupid." They're always behind him, asking questions, trying to catch up. Now, bringing the show back, they could have made a super smart companion, one who could talk to the Doctor about nuclear physics. But a) how boring would it be to watch a bunch of science nerds chattering incomprehensibly every week, and b) she STILL wouldn't have been as smart as the Doctor. So we got a companion who was average. She was inquisitive, but not to the point where she made the Doctor spell everything out. She didn't take bad guy plots lying down, but she knew to appeal to the Doctor because he was smarter than her--thus keeping both characters integral to the plot.