I've definitely noticed this, especially being on a college campus. When I look at costumes, I would love more options than looking like a stripper or swimming around in a costume made for men. Why aren't more modest costumes that are also flattering available?
That being said, sexy costumes do have a place. There are a lot a women who look forward to the opportunity to be sexy on Halloween, and I fully support that choice. I just wish there was a wider range of options available for women.