Ichika is open and appears competent and womanly, even if she's an untidy mess in reality. She's approachable.
Nino is assertive and intimidating, which scares boys off.
Miku is enigmatic, so people would be hesitant to approach her.
Yotsuba is virtually a whirlwind of energy, which gets exhausting.
Not sure why people don't approach Itsuki, though. Maybe she seems too proper and formal.