I agree. Though katawa shoujo is not exactly the best visual novel, it is certainly an important factor in controlling the world with visual novels.
As for why it is popular, it has to be it's accessibility. Compared to other visual novels, this one is very easy to download and start playing/reading. no patchers, torrenting and all that stuff. It's got a fairly small file size, and it also has an all-ages option.
Pretty much no reason not to try it out, considering how it is free as well.