1)cause sasuke was cuter as a little kid
2)he has better hairstyle
3)itachi didn't kill his entire family
4)Sasuke is hard to get
5)When I first saw Itachi i thought he was like 40 something
Need i say more?
This is how it happened. The women looked at Itachi and had their minds blown by his superior sexy. They never recovered mentally and they fell for Sasuke instead
Sasuke looks alot better and Itachi seems like he wasn't around too many people all day. He was a very quiet person, worked for Anbu, probably wasn't noticed by the girls. Sasuke was always good at school, very handsome and all girls liked him. Maybe also because he was arrogant and they competed for him.
Where did you get the information of Itachi's girlfriend? Now I'm curious about what I missed. Please tell me.