When a professor is rated low, at least some of that rating has to do with the professor having a high standard in the awarding of grades and passing of students.
If students get good grades then no matter how much the lectures suck, or how unavailable that professor is for help and so on, large numbers of those students will not rate that professor low. Poor grades are the exacerbating factor in a revolt against a professor.
If you're a C student, the moment you see a B+ next to your student ID on the course results table, all memories of the prof either vaporize or become rose-colored.