Going back to the "past" chapters, she doesn't want iris in the club because she was incredibly upset at people joining her club just for her. she's hot and popular so people joined for her, not because of their love for mystery. Iris is the same, not really a fan of mysteries, but loves yuriko, also she's probably overprotective of her in some way.
of course you can argue toiri and himawari also dont care for mysteries, but yuriko is under the impression that himawari is actually amazing at solving them, and toiri is also good at it. also, i do think toiri does like mysteries to some degree. where as iris had neither the interest or the skill. whether that's enough to make this feel satisfying is up to you
@Elki