Because the most critical part of the U turn on 320 is positioning the left seat in the case of the right turn (or the right seat in the case of the left turn) exactly above the runway edge and that is very hard to accomplish if you're trying to put your significant cockpit other over the edge, and not yourself.
As for the visibility thing, you usually make U turns on the runway and assumption is that noone is tailgating you. If that assumption is wrong, ATC will have a lot of things to explain.