From her point of view, she's afraid he might convert to Angelus, she wants him to be attached to the world and she cares for his happyness. She sees him as her boss, even though she sometimes do the bossing. She never approved on Buffy and Angels relationship because she feared the world would go down. Why would she then fall for him? Tell me! From his point of view, Cordelia's pretty much like Buffy. She's mortal, deserves to be out in the day, to have a future, kids and all that, just like Buffy. He left Buffy because of the things he couldn't offer her, why would he then even concider a relationship with Cordelia?
An other thing that pisses me off in the whole Cordelia vs Buffy deal is how Cordelia is the one doing most of the bitching and also receives most of the awards. She's Connors mother figure when he's born, a role that would more naturally be Buffy's. I know that to have that happen in the show would be impossible with them being appart, but I still think of it as Buffy's role, not Cordelia. Cordelia ascends to a higher place, as a gift, what gift does Buffy receive? Death. Something doesn't quite measure up here. Buffy's the one of the two most deserving of something good, but she always gets the short end. Buffy is their warrior, not Cordelia. And to mention it again, I don't hate Cordelia, I love her, but I disagree with the path the writers took with her.