Why does meat in the crockpot always end up dry?
Cooking anything really lean for a long time can result in super dry meat.  Chicken breasts have little fat and little connective issue.  The same goes for a lean beef roast.  You can do chicken breasts in a crock pot - just not all day, more like a few hours on low.

Try something with more fat like a thigh (or any dark meat) or a beef chuck (shoulder) roast and you should have more luck.