On Discover Channel's "A Haunting", why are the families that are haunted are always Christians?

Because some Christians believe in demons they make a big deal of a spirit that comes along while other people don't;

I'm guessing this is a big one.  When I first decided to move my kid's bedroom upstairs, it was still an attic with a ladder, no lights...very spooky.  I hauled myself up through the hole in the floor, and was almost immediately the recipient of a big "Poltergeist" style bombardment with "Get out!" (not out loud, but very clear in my head) and just general purpose threatening feelings.

Me and the spook (who we later found out was named Ted - a grumpy old dude and kind of a perv, but basically harmless) had a little discussion (okay, I talked, he sulked) on the subject of how this was my house now, he could stay if he behaved, and don't start with me when I've had a hard day.

He wasn't a demon or even a bad ghost, really - but if I'd been scared, I might never have gone up there again, or worse, I would have gone up there while scared, and fed him.  So Mr. Ted would have gone down in history as a bad ghost, or even a demon, instead of sort of an ectoplasmic PITA but  basically harmless.

At the time there was also the spirit of a very timid elderly lady here who actually warned my daughter about Ted - apparently she (the ghost) was scared of him!  Which probably gave him a big head, which probably contributed to his thinking he could scare me off.

Ted hung around and pestered off and on for a number of years...my kid had a boyfriend that he especially liked to bug.  (Kid was raised very strict Southern Baptist, *and* he had quite an ability to sense/communicate with spirits - a bad combination).

After John died I didn't see any signs of Ted - I think John didn't want him hanging around me, all the "spooks" seem to have departed (except John, of course).