"Why" is kind of a useless question here, I believe. I don't mean that to imply that you don't have valid opinions on the matter...but the "why" of it doesn't matter, does it? Some people do it because there is a market for it, as has already been pointed out. Some people genuinely love these animals and breed them for the sheer challenge and joy of it. Some may see it as some sort of conservation effort. Different breeders are going to have different reasons and motivations.
The real question, perhaps, should be: How can we make sure that all giant snakes in the pet-trade end up in the hands of responsible, educated owners? No easy answer for that one, I'm afraid.