How to "bake" whole potatoes in slow cooker?
How frequently did  you lift the lid to 'look at them'? While water is not required convection is, and each time the lid is lifted all the hot air does exactly what hot air does...rises and escapes. The slow release of heat through the ceramic sides do not reheat the space quickly and if one peeks too often (or doesn't have a good lid), you will get the results you report, over done on the bottom, where the wrapped potatoes are in direct contact with the ceramic, and under done on top.

If not only from my experience doing so the sheer number of posts from people reporting success with 'baking potatoes in a crock pot' says that 'it is possible'. From comments above I believe you may have also managed to overcrowd the pot. I have never tried more than four 'good sized' potatoes at once and always had good luck. A quick scan of the articles offered from a quick google search (above) has mixed suggestions, from only cover the bottom (what I do) to fill the crock. Try again with few potatoes, in a single layer and keep the lid sealed.