i think qiu's appeal to most people is that they're just generally more charming in a way. like the previous comment said, it helps that they're open. qiu is naturally a doting person in step 1 who'll go out of their way to look out for the mc in several occasions (at least from my game experience). they'll actively seek you out and look out for you – inviting you to walk to school together, inviting you to join the boys' club, and even more depending on certain dialogue options. i think having the chance to be doted on is comforting to most, and that being qiu's default would ultimately turn them into a fan favorite rather quickly.
that's how i view it at least :]