Because that makes no sense and don't give me that well they didn't show them using the bathroom so its possible stuff, the audience isn't so stupid they would assume people don't pee in the future if it isn't shown where as a promotion has to be mentioned or it or noone knows about it and it probably didn't happen.
Plus if Sulu was going to be a captain it would have been sometime after they got back and that would have gone out the window the moment he helped Kirk steal the Enterprise no matter what the Federation president said.
And seeing as the cut Sulu promotion scene had ADMIRAL Kirk being the one to cut the orders, well guess who isn't an admiral anymore and as such has orders that now don't mean crap?
And forgetting all this name one thing (outside the novelizations of Treks II-IV) that points to Sulu being a captain or about to be promoted to captain besides a cut scene that probably dosen't even exist anymore.