Because Abby fought with Christi, and Abby told Christi to fix Chloe's eye at their last competition.
Because Christi and Chloe were no longer close to the rest of the people on the team, either.
Because Chloe was becoming a worse dancer at the ALDC.
Because they thought they would be better-off elsewhere.
(Outside filming for the show, Chloe had already left the ALDC after the third season - which doesn't mean she was training somewhere else, but might make her better described as an RDP rec student. In the fourth season, she filmed the show without signing Abby's studio contract, and without doing ALDC competitions that weren't filmed for television.)