Completely shower-thinking here, but you know how the Mystery Research Club members all have some kind of gimmick? Hypnotism, Magic, Occult /Paranormal activity, and science? What if Daisuke's mystery was literally being a harem protag? Because he's not exceptionally nice, nor does he have any sort of weird quirk, nor does he have any past history with any of the girls in his harem--he's probably the most "normal guy" character I've seen in this kind of manga in a while. And yet, every so often, he continues to get additional girls who want to be around him/with him.
Tl;dr: Daisuke's Mystery Research is Spontaneous Harem Protagonist Syndrome.