The person who invited Tomori into a band hold special place in Tomori's heart. In CRYCHIC case, it was Sakiko. For Mygo, It's Anon.

That was reason why:
1) CRYCHIC: Tomori quit/stop coming after she learnt Sakiko won't come back anymore. She didn't care that Soyo, Taki and Mutsumi were still there.

2) Mygo: Tomori was still coming and had hope when Soyo and Rana were gone but completely broke down and quit when Anon left. She didn't care Taki was still there