The incentives are there from other clubs. The PL and UEFA are under pressure from other high profile clubs and bodies to clamp down. If it wasn't in their interest they wouldn't exert so much time and money in trying to enforce FFP in the first place.
From what I've read the difficultly actually comes from the courts and legality of the punishments. The few times UEFA have tried to punish these guys it's either reversed by CAS or at the very least the punishment is watered down. There's also a matter of proof, it's all good and well alleging something, but proving it to the extent that it's waterproof isn't easy.
Lastly though, FFP has actually worked to a large degree. For the most part, clubs are having to limit themselves to the extent that they are. They now just need to tighten up on the loopholes or outright cheating.
