Why do I keep landing on my knees and head when I do a back flip on a trampoline?

Because you are doing it wrong and you don't have the aerial sense or control to choose a safer landing even if you under-rotate. That's why you shouldn't be doing it.

You need to take some proper lessons and you needs to build aerial awareness and control through doing lower grade moves like seat, front, and back landings, with and without twists in and out, and transitions between said landings with and without twists. You also need a really good back landing to build the actual rotation mechanics of jumping up and pushing your hips forwards. You are almost certainly doing it fundamentally wrongly, as pretty much every person I've ever encountered without proper training does. People tend to chuck themselves backwards which is highly dangerous.

There's just no way to make what you are doing safe or effective without proper training.

Fredo you simply have too much butt and it obstructs your jump. Trim that rear or take up another hobby.