Crunches vs. Sit Ups

http://www.fitsugar.com/Tone-Your-Abs-Without-Crunches-7859508

I honestly don't know if it's just me but crunches are awful-- like painful for my back and neck. I do proper posture and everything yet they are one of the two most uncomfortable moves for me to do (second most uncomfortable is side leg lifts when you're laying on your side on the ground-- my hip bones just shove into the ground like a bruise). I can do sit ups just fine and I can still challenge myself with those too. I feel my full midsection working with abs whereas all I feel is upper abdominal working and my neck and lower back yelling. Maybe that's why some trainers combine the two-- to isolate the top them make it all work.
WHY do people prefer crunches over abs though when they're only doing one? Is it because it's a burst workout? Is it because they can't do sit ups?
Until I figure out why people are doing crunches instead of sit ups, I'm going to keep doing sit ups so I can spare my poor back and neck. I have no problem with other abs moves either. The link above gives plenty non-crunch moves.