Because of what's left in the stores in my size when I do dispose of the magic combo of time and extra $ to shop.
I am a petite size 0 or 2. Extra small in some brands, small generally. And here in Canada these sizes are always missing first! It gets frustrating. I was told it is because they order very few of them.

I am then left with choosing among lesser options. Add to this that I don't have conventional body proportions (smaller, chubbier legs, long torso), and it reduces my choices even more. Sometimes I get discouraged and I buy something I like, even if it wasn't what I was originally looking for, just not to come home empty handed (because occasions to shop are far and between). But then it sets me back in my efforts towards a personal, coherent style. Because I blew my allowance on this particular piece, I must wait a few months before the opportunity comes up again. Not to mention that whatever I bought is often something on sale, which is more likely to end up as an orphan.

Oh, and I hate shopping online. It reminds me of how wooly-itchy stiff clothes were ordered and shipped to us from France when I was young (and I hated it! I wanted to dress North-American). It's not convenient here, not being able to return items easily and all. Finally, I need to see the item on me: I have such a strangely built body, no online model looks like me, so there is no way to tell how the item would drape and appear.

For me, there are few different issues too, which are kind of interrelated:

1. It's just hard to find a huge variety of clothes that work for my very short, very curvy body. Which means I tend to end up going for the same brands and styles over and over again because they're easier and I know they'll probably work.
2. Which brings me to: I'm really lazy and comfort-driven. So going around to try on a whole bunch of things on the off-chance that one of them might work doesn't sound appealing. Neither does getting everything hard-core tailored to fit me. And I don't like constricting materials/outfit combinations, which limits me even more. I seriously even find wearing jeans annoying because they always feel stiff.
3. I'm also very indecisive. So a lot of stuff looks sort of cute to me, but very little really calls out as something I love.
4. My aesthetic sense isn't particularly sophisticated and tends mostly to respond to very obvious cues, like pretty colors or prints, as opposed to subtle details of cut or texture or whatever. So a lot of what I buy tends to be repetitions of the same few basic items, with the color/print being the only real interesting part.

I also face a lot of the same budget and fantasy vs. real-life concerns other people have mentioned.