Why do diabetics get cracked heels?

I believe it's because they have poor circulation, which creates lots of problems in the lower extermities.

For non-diabetics; massage oil from health food store and socks - wear while sleeping. Vasoline works, but I do not like putting petroleum products on my body.

The poor circulation is also compounded by peripheral neuropathy, which means that they are less likely to feel a small wound or irritation at first and so it may get worse, whereas normally any pain causes a person to favor the foot, see what's wrong, etc. All of these issues are secondary to chronic high blood sugar, although it's not entirely understood how that has all the effects it does...

Theabk is right on. The combination of reduced circulation and reduced sensation can result in all sorts of problems, including Charcot's foot, where the bones of the foot degenerate and collapse. Diabetics should develop a routine of regular Visual Surveillance of Extremities checks (VSE) to visually check the physical condition of all areas of their body. I have found that even one week off my diabetes medication results in even the most minor and superficial of wounds failing to heal properly, in fact, a few weeks ago my prescription expired and before I got it filled, I had a scary but fortunately brief bout with cellulitis in one calf, resulting from a minor insect bite that failed to heal properly. Within hours, the infection (which was several inches from the bite that started it) was spreading rapidly, hot to the touch, and extremely painful. Fortunately, I recognized the symptoms and immediately got medical attention. Waiting even a day or two on something like that (which at first seems to only be an unexplained and painful bruise) can lead to them taking pieces off of you if the infection gets beyond antibiotic control.