Because dragons are solitary and territorial and don't want "friends" in their enclosures. Males in particular should never ever be left together, or females of differing sizes. Even if a pair of dragons act like they're getting along by human standards, they're still playing domination games with each other- People think it's "cute" when they lay on top of each other like they're snuggling, but it's actually a domination thing. People will tell you "Oh, I've kept mine together and they're fine", but I've never heard one of those stories have a long term (as in, dragons entire life up until completely natural death, so 8+ years) happy ending. I'm sorry you had to find this out the hard way.