I think alot of it has to do with the how the bands set themselves apart. Most christian rock is soft and generally all about loving god, jesus and all that good stuff. The vocals are very clear in those songs and the messages seem very overwhelming to people who aren't into it.
Take black metal on the other hand. Alot of the people listening to it aren't into the satanic messages and all that crap. Take me for example. I like Mayhem, the notorious devil worshipping, church burning group that used to play with pig heads on pikes as part of the stage show. I'm not a huge fan or anything but I can appreciate the music because of the atmosphere it creates. I can't even understand the vocalist so I'm not getting sucked into their stupid anti-god crap when I'm listening to their music.
That's not to say there aren't any good christian bands. One of my favorite bands is a christian thrash metal band called Tourniquet. I also enjoy the occasional Switchfoot song and Flyleaf isn't all that bad.
So I guess what I'm trying to say, in short is:
Christian music gets no respect from people who listen to rock music because the religious messages can feel a little bit like an attempt at conversion (it doesn't help that most mainstream christian bands aren't very talented).
Death and black metal gets its share of heat from religious/parental groups because of the gory/anti-god messages. However, the reason it's not (often) scoffed at by your average rock/metal listener is that alot of people can't understand the vocals and just enjoy the instruments (and occasionally the vocals, when they actually add to the music). The message isn't often at the forefront of the music.