It's definitely not the lack of uniform that's to blame! In terms of school management, though, uniforms are easy to specify for blunt enforcement. And then there are all the arguments about preparing kids for the "real world" (where I see hardly anyone dress like that!) and, when people are honest, grinding down their individuality and spirit.
Two more 'justifications' that I have heard surprisingly often are "It's tradition" and "The kids need it." I don't know what either of these mean; people who assert them don't seem to understand my request to unpack their claims. I have a feeling that they find these views obvious and self-evident in ways that I don't.