Fuutarou has gotten the girls to start liking him due to different aspects of his personality. His "niceness" and steadfastness helped to open them up to him, but it's not the reason why they're all falling in love.
1. Ichika enjoys bantering with him and how he's also hyperfocused on the future like her.
2. Nino hasn't fallen for him, but while she has a thing for bad boys like his younger self, she's going to fall for his family orientated side, how he cares deeply for his little sister and father. She's a mother hen, deep down.
3. Miku loves him because he's one of the few people that tried to break through her shell with the things she loves. Sure, you could say that's simply being nice and not that special, but Fuutarou was the first eligible boy who do so, which makes all the difference.
4. Yotsuba is a mystery wrapped in an enigma and layered with paradoxes.
5. Itsuki is the only one who shares Fuutarou's drive to excel academically. .The others just want to pass, but these two are the only ones who are about succeeding, that is, getting more than the bare minimum.
The fact that Fuutarou is a witty, handsome and suave boy who is in almost constant close contact with them all just clinches it.