^^ because everyone has their own preference and wants their hair/style/fashion/appearance to convey certain social messages
i think the queer thing i just mentioned is a great example of this -- imagine you're a nonheterosexual woman who feels most comfortalbe in skirts, heels, makeup, long hair... there's a frustration that comes along with being 'femme' that is not being VISIBLY obvious as a woman who is into other women. having part of your hair cut quite short is one of those 'indicators' of things that are 'more common among the queer community' while preserving the longer hair and 'feminine' appearance that's more comfortable to you -- of course this isn't always the case and by no means says that this haircut means you're gay or anything!! just using it as a (personal) example of a social motivator for an 'ugly' style.
there's also this other thing i know i've always done which is to deliberately avoid being 100% conventionally pretty, because i don't want to associate with folks who are only into that, and while i've toned it down as i grew up (no longer 'need to make myself look the opposite of the popular high school kids') i still am definitely more attracted to things which are not 'beautiful' and that are darker, edgier, more 'conceptual' or 'reference' things rather than being 'pretty'.
there's plenty of 'not-pretty' looks out there that people love because of their social indicators or references -- black nail polish, more goth looks, tanning themselves completely orange, unusual hair cuts and colours, giant dinosaur claw heels, white-painted faces, practically all couture -- and that's what makes fashion all so interesting