Because it was easier, easiest or rather cheapest way, uniforms are all the same for everyone on the same show, but it's only logical to me that a Captain may be able to choose uniforms for it's ship. That's one thing I like in STO. Yes, all uniforms look like starfleet uniforms, they all have some distinctive look to them, but there are lot of different versions and options in colors. It's a way to distinguish one crew from another on a big stations.