No real challengers ran b/c she created a sense of inevitability and challenging her would have just resulted in you being a failed spoiler that harmed the party's candidate. I think you'll see the same thing happen in 2024 in the GOP if Trump runs.
But, I think you underestimate how much Democrats like Hillary. She was very much qualified and competent enough to be president and I think she probably would have done pretty good. Easily better than Trump but that's a low low bar given his historically bad record on jobs and deaths.