What in the world is "kawaii lolita"?
(also, never mind that gyaru is associated with Shibuya, not Harajuku...)
I've seen people wear manba and decora at the con.
I think people wear lolita for a few reasons. One is the obvious (and incorrect) association with cosplay, and the fact that they often sell lolita (or "lolita") clothing at the con. Another is that there is a sense of community, since there are meetups and panels and a lot of the lolitas know each other. Another is that it is something that might be a little too much for everyday wear for people, so they wear it to events like conventions, or that they enjoy putting together really out there and over-the-top outfits that they wouldn't wear normally but can get away with at a con. It's also quite popular, and growing in popularity, especially since there are now multiple brand stores in SF that the clothing can be bought at, in-person, year round.
Gyaru fashions tend to be more of an everyday thing, so while I'll see the occasional person in, say, a Liz Lisa-styled dress and everyday gyaru makeup wandering around because that's what she wears, a lot of the time, the gyaru that I see are wearing something much more outlandish, like manba, for the same reasons why people wear OTT lolita at cons (they can't get away with it in daily life and want to wear something crazy for the event). Gyaru also tends to be relatively normal as everyday clothing goes (relative to something like lolita, which can be and is an everyday fashion but is a bit more out there), unless it is one of the more outlandish styles (like manba or even full-on JD-styled hime...which I've never seen at a con, haha). Even though manba is outdated, it still gets revived every so often in the magazines just for fun and would probably be a lot of fun to wear. I don't follow gyaru too closely, but I follow it some, haha.
As for decora... I think that one's also a bit outdated, but I still see it around at cons a bit.
I bet that now that fairy-kei and mori girl and some other styles are becoming more popular, you will start to see more of those fashions around cons.
I know that for me, I tend to wear lolita at cons for the events and the sense of community, and for the somewhat fancier outfits than I can get away with in daily wear (since I tend to wear somewhat toned-down lolita everyday...ah, the joys of being a college student and being able to get away with that!). Plus, it's nice to change into something cute once I'm tired of wearing cosplay all day, and why not a fancier version of my normal clothes, right?