1. Because weak and tight muscles allow bones to move out of alignment.
2. Because of hand and foot dominance that repeatedly twists our body in the one direction every time we hit, throw, kick ....
3. Because of the way we sit down (in the 'slump dog' position) for hours, weeks, months, years and decades.
As muscles attached to the pelvis become shorter the pelvis moves out of alignment. Then the bones above it move out of alignment. You think you've got a lower back problem when actually you've got a muscles-attached-to-the-pelvis problem.
You rush off to people who want to X-ray the spot where it hurts and then crunch, rub, heat, vibrate and palpate your lower back, instead of YOU, lying on the floor while you watch TV, doing the exercises that gradually loosen off the tight muscles.
4. Because we don't have a regular and systematic strength and flexibility training program.
If everyone did yoga a few times a week (and had a strength training program at the gym) doctors, radiologists, physiotherapists and chiropractors would be sitting around twiddling their thumbs and playing golf most afternoons.
Now read on:
In the mean time stay tuned, highly tuned and strengthen weak muscles and loosen off tight muscles.