Because for some weird reason, only mortals appear to be capable of utilizing artifacts for good and evil. I think it ties into some kind of balance and choice thing in the struggle between God and Satan.

Or maybe, in the words of Dark Helmet 'Good is dumb', and Evil ain't much brighter.

Possibly cause the demons find they are allergic to power ups, and instance death may rise should they touch them.

Because they actualy ARE demons who trade their powers for possessing you without your knowledge! And wow, Reminds me it's been too long since I've last fired up ZDoom, Still the most timeless game of all time in my book. The true wonder of modding with ZDoom is how advanced it is while remaining dead simple! I've tried modding other games but had me beat by light years. Long live Doom!

Long live Doom. ;)