Grazere said: I'm pretty sure the main reason is that Charlotte's little self is cute. If she was a bucket of slime, this wouldn't be happening.
Adding to that people's various speculations on Charlotte's fixation on cheese.
If she was a bucket of slime, people would probably have attempted to deduce how she looked like before she was a witch and then proceed to make doujin out of it. lol
Besides, since Madoka re-wrote the rules of universe it is now perfectly possible for them both to end up in Madoka's Valhalla one day.
No!No!You've been alone for a long time too, haven't you...A manga where Mami-san lives happily in Charlotte's world.Chocolate castle: complete!dere dereshiver shivertsun~I'll pretend I can't tell what you really feel...