Long reply warning, but two reasons I guess. The first one being that the keyboard I use the most, an IBM Model F XT happens to use the ISO layout, and I've found that switching between my other ANSI keyboards can be a pain. So I'd prefer to just stick with one layout for simplicities sake.
But also, I've found that the shift and enter key just feels much more natural to me. The enter key is very easy for me to find with my pinky, and the same goes for the shift key. I also prefer to swap the CTRL and Caps Lock keys whenever possible.