i think the basic idea is that gotham was already riddled with crime. but they had the old-school gangsters. mobsters and stuff like that. and they were always there. but then batman starts taking them down and instead of other normal mobsters taking their place, the more garish villains start to arise. i don't know why there was a change in the mental health of the villains of gotham. but i know that they began to replace the old form of criminals in gotham. that's the best answer i can give you right now.
but it's possible that they feel this is the best way to make a name for themselves. gotham is a big city and batman is well known. so fighting, or even taking down batman will get you fame.