neurotoxic/hemotoxic???

why are more old world snakes more often neurotoxic and more new world snakes hemotoxic?

My personal opinion on this is that venom is actually an exaptation and was originally used as a defense not one of offense or prey abduction. The way I think about this is that looking at a neurotoxic venom for example we always hear of the Cobra having enough venom to kill an elephant. Now granted this like an LD50 study and damn near irrelevant but the fact remains if that snake is eating snakes why the hell does it have such a powerful toxin to abduct prey? Fast acting I understand, the lethality is what seems off to me.