Why can angels go to hell, but demons can't go to heaven?

I'm larely agnostic, but this question confuses the heck out of me. Bare in mind that I really don't have much knowledge on this subject and am not trying to insult anyones faith or lack of. I just know that angels have fallen from God's grace and been sent to hell, so how is it that a demon cannot become good enough to go to heaven? My second question is why are all angels pretty and demons are always so ugly? Can it not be possible for a demon to be beautiful?