For starters, he's not nearly as unpredictable as he'd like to think. He has a tendency to pick laughter themed buildings as hideouts, making him easier to find than a villain who's less discriminating in his choice of lairs.
Also, his ego.
In this episode, a man plays on the Joker's ego as part of an insurance scam. If Batman hadn't gotten involved, the whole scheme would have worked perfectly. He never even suspected the Casino's theme was deliberately chosen to upset him. In Return of the Joker, he really loses it when Terry McGinnis starts taunting him back. He really strikes a nerve when he says that the Joker never made Batman laugh because his jokes were lame. If you want an out-of-character explanation, basically, the Joker only survives due to author fiat. Put simply, he stays alive because no one ever realistically includes the ramifications of his actions. He's murdered any number of people, caused widespread property damage, broken countless laws and (in the animated continuity) even threatened Gotham with a nuclear bomb! In any truly realistic portrayal of the Joker, the GCPD would be ordered to shoot him on sight, and various government agencies such as the FCI, NSA, and DHS would be doing everything in their power to kill him. However, no one has the guts to really kill off the Joker, so they just throw him in Arkham, which, judging from the rate of escapes, must be made of balsa wood.