Why is it that I can do sit ups with ease, but find crunches extremely difficult?

Your lower abs are stronger than your upper abs. Sit ups test the raw strength of your core. Crunches focus on your upper to center abs and keeping perfect form test the muscle's endurance. You get a tiny break doing sit ups when your back touches the ground; with crunches your abs have to stay tight the whole time while you're moving. Hope this helped!