Hina and Parsee, at the very least, have their own sort of associated quirks, practically ascended to memes, which is more than what some people can say. I have nothing for Kyouko though, but the lack of attention could just be because the game's the newest. Of course, there was quite a burst of interest when the character was revealed, but I suppose that's standard. And in any case, what's wrong with the ⑨ squad? From what I see, their grouping as a bunch of ⑨s isn't really a bad thing, and lots of people consider their childish behavior a lovable trait. Mystia's got her own little niche as well, as...Yuyuko's food. And finally, just as an addition to what you were listing earlier, don't forget Cirno (speak of the Scarlet Devil) and Chen, who've got their own respective spotlights from the fandom.
TL;DR: They're not really getting little attention or thought of as unpopular, considering how important they are. Also, I'll just leave this here. In particular, the results for character poll #8 may be interesting.