Probably just from the feel and maybe the odd look. I couldn't justify any reasons besides those two.
I'm not sure if all of them have horrible mag placement, but having the mag behind the trigger to me is an odd feeling, especially since I'm right handed. So having to go behind my right hand or use my right hand to change the mag would just feel awkward. I'm not saying I couldn't get used to it, just prefer not to. Also my biggest thing is if SHTF scenario, ar15 parts are way more common than bullpup.
A case in point in why I love my two Glocks. I love my FN Five-seveN as well, but if something breaks on it then I'm literally screwed.