Carmen was the woman that made Evan realise he can enjoy a woman's company without sleeping with her (this is what he said, even if he had enjoyed Alex's company for three years living in the same apartment before Kelly moved in).
She broke up with Evan because she didn't love him the way he loved her. He's also younger than her, which strained her a bit because she didn't feel up to staying out as late as he did.
Carmen also has two children that she had to take care of. Evan loved her children too, but she still preferred to take care of them on her own.