Because people are morons. Riri makes a nice, smooth, zip, that I find really nice on say, a purse, or a zip pocket on a pair of pants, but ime. hgg end YKKs are much sturdier, and I prefer them for things like bags and outwear. Brands like Lampo are also nice.
One of the big problems with Riri is that all the zips are made to order, so a broken zipper is impossible to replace, which, for me, at least, is a big negatve. You have a YKK number 10 heavy outerwear zip break, and you can get it replaced at your local guy, no problem.