Capes get a pass, I believe, because they don't really have a history in America. It's basically a borrowed tradition so it was never "fashionable" here in the first place. A better question is why they're still accepted. I'll point out that most new characters don't actually wear capes. It's very rare these days. So its less that capes are accepted when trunks aren't, and more that capes aren't ridiculed like trunks are. And boots.....well no one says boots are outdated as I suspect many of us (at least on the American east coast) are wearing them right now.
Trunks are considered outdated because they literally are an outdated fashion that you no longer really see in American culture. Not even wrestlers really do the trunks thing much these days (at least as far as I know, I haven't followed that in a long time). Combined with the running joke that the trunks have been in popular culture since.....the 60's at least, and it's little wonder why people don't respond to them.
Now that is just silly. Perhaps people who don't like the trunks don't like them simply because they don't care for the style. I don't like my female heroes to look like they're wearing bikinis. Does that mean I hate beaches or scantily clad women?The dislike of such a quintessential part of the design stems from an insecurity about liking superheroes.