It isn't the fish themselves, it's the behavior of the people around them. They are sold by unscrupulous/ignorant dealers as purebreds of a given species essentially ripping people off either by accident or on purpose, some get released into the wild with a negative effect on the local fauna and sometime the flora as well (bad enough that purebreds get released but this "creates" all new fish to add to the problem), there are diseases that arise in hybrid lines that don't have the same strength in purebreds (note the recent yet rapid rise in the condition that seems to mimic Popeye+cancerous ulcers at the same time that seems to affect Flowerhorns almost exclusively), it gets harder for people who want original breeds to find them in lfs because those tanks are more and more loaded with Flowerhorns, Fake Parrot Cichlids, Balloon Mollies and other such oddities, and it also fosters an attitude of "I'm gonna stick these two fish together just to see what kind of babies they make" which isn't exactly responsible pet ownership.
Those are the things that come quickly to mind, but I may have missed others. Hybrids should not be spread through the hobby simply for the sake of a pretty/aggressive fish. Keep 'em home; in nature they would make great snacks.
