Most people have the explanation down in literal terms. (IE: Kiyohiko Azuma was appealing to the lonely geek demographic) but applying suspension of disbelief? We still get a logical story. Think about it. What are the girl's traits?
Tomo you would think might have a BF, but her hyperactive personality is a turn off because guys will view her as attention hungry.
Osaka? Sorry, but teenage high schoolers tend to avoid girls who SEEM to be self-interested. Osaka is not self obessesed, but she's got ADD up the wazoo. We have the other end of the spectrum here, people will think that she wants NO attention.
Sakaki? It might seem strange at first, but it isn't when you think about it. She's popular with the other girls in the school and a bunch of the athletic clubs, but the fact that she's not actually in them or seen dealing with them often creates an image of elitism. An image the series even clearly shows she has on numerous occasions. Regardless of whether or not she feels that way about herself.
Chiyo? Obviously, she's too young.
Yomi may be a geek, but lots of geek girls get boyfriends with ease. In all likleyhood, she has no interest in dating. Has she had requests? Inevitably. But lots of attractive people in high school go through all 4 years without so much as holding hands all because they have no interest. This isn't strange.
Kagura is elusive on the subject. She doesn't seem like the kind of person who would turn down a date. Being a tomboy is far more likely to BRING attention then turn it away. She plays fucking video games for christ sake!
The other girls have some explanation, I can't think of one for Kagura.