I personally think she left because the pressure was getting to her. Korey kept saying he had to keep having meetings with her (and Ray) so she could find her role on the team and develop more of a complete personality on the show so he could justify paying her. The casual call-in show was in flux at the time, Goodwin had gotten replaced as host, Ray and Goodwin had been at each others throats on the air for weeks, and people were still criticizing Shea for not bringing anything to the table aside from "being a girl" and answering relationship questions, and sitting out conversations related to any nerd shyt.
For months Korey said Shea needed to "find her voice". I think she realized Korey was asking for something she couldn't give him. She was trying her best but simply wasn't as entertaining as someone like Danielle, who had just started coming on the show as a fill-in, and brought a personality to the show that was more than just being a girl. She also had some long distance shyt with her boyfriend so it's possible she wanted more time away from the show for him but I dunno if my speculation can go too much deeper into thinking that was the reason why.