Actually, most of the wigs I've seen are either shoulder length or pixie short, and always straight. I've seen a couple of women wearing wavy falls, but that's pretty unusual, and it's the curliest I've ever seen.
There are a few rabbis (ultra-Orthodox) who feel that long hair isn't properly modest, but most rabbis don't have a problem with long hair or curly hair. I think a lot of it has to do with ease (shoulder length is sophisticated but easy) and conformity.
"There is such a thing as natural beauty, but every woman needs a little help to bring it out."