This Newsweek article explains why...

"We stopped teaching people to do crunches a long, long time ago," says Dr. Richard Guyer, president of the Texas Back Institute. That's because the "full flex" movement--the actual "crunch" part of crunches - puts an unhealthy strain on your back at its weakest point. The section with the most nerves (and most potential for nerve damage) is in the back of the spine, which is the very part that bends and strains during a sit-up.

My observation on this is I bloody knew it. I'd started doing crunches at the gym again and it's been hurting my back but I thought that was just being out of shape. I have a scoliosis (a curvature of the spine - it's relatively common) and my back is particularly sensitive. I had thought by working that area I would be helping my back.

As a historical side note, I used to 300 crunches each night when I was in my teens and I don't think it did my back any favours.

The link suggests other exercises to target that area to build up abdominal support, including planks, and pushups. I reckon yoga is probably a great solution for building core strength in a non-damaging way.

Anyway, well worth a quick read: Newsweek article - Stop Doing Sit-Ups: Why Crunches Don't Work.