Simply that it's cheaper/lower-end wine. It may lack some of the more complex flavors that more expensive wines have and that some people "require".
It's kind of like drinking KoolAid instead of soda - soda has a bit more of a refined flavor and texture (due to the carbonation), but that doesn't mean that KoolAid can't be a tasty, refreshing drink.
It's just a stereotype. A lot of wines that come in boxes also have bottled products, but they're all cheap. That's okay though because if you like a wine, freaking drink it. Only asshole wine snobs will judge you for it. I get boxes because it's cheaper and less wasteful.
When it was first introduced, it was CRAP!! They have made a lot of advances with the bags, and air-permeability, and 'keeping quality'. It is a lot better now, than 20 years ago!! Besides, a respectable girl can have a wine in a box, and not feel guilty, drinking too much on a dateless Friday/Saturday night, because it isn't like an empty bottle!! You can only tell it is empty, when nothing comes out!! :) No offense intended!! :)