There was that fellow, wasn't there, who commented over several pages on Amazon that Lundy did nothing for him, an abuser who wanted to stop, because the book didn't answer the title question. My thoughts were that it was a pathetic justification for continuing to abuse. If only some wise author would tell him, the reader, exactly why he was doing what he was doing, suddenly he would see the light and be able to stop. Er, excuse me, CRAP. It denies all responsibility for his own actions. Was that man really incapable of asking himself what he got out of abusing? Don't believe it. The book tells him a whole lot about how much women don't want to be abused, the behaviour that just won't do, and how an abuser can learn to stop. None of this was good enough for our amateur reviewer. He wanted The Reason. The magic bullet. And that pretty much explained why he was an abuser, with an attitude like that.
I actually thought the book did answer the question, though more implicitly than explicitly. He states it is about power, control, and entitlement. Take the parable about the boy who was told he owned the park, for example. Basically the abuser has been brought up all his life to believe that he will have a woman to serve his needs because that's what they're for, and then gets such a nasty shock when it turns out not to be true that he may never adjust. He needs to let go of the belief that he is entitled to behave that way - accept the fact that he does not own the piece of land after all, or the woman. And that's hard.
Lundy doesn't seem to buy the theory that abusers are insecure (and George Simon in "In Sheep's Clothing" categorically rejects it). I don't know if this is always true. I suspect, for example, that a lot of it was about insecurity in XH's case and probably also in my dad's. So I don't 100% agree with the answers, but I do think they were there.
Had the book come out more clearly with "this is why", you can bet yon abuser fellow would have just said "well that's not why I do it, so the book is a load of twaddle and I'll just have to carry on acting abusively, that's all. Oh why will nobody help me?".