Carriers leave alliances for various reasons. EI used to be in oneworld but left when they went LCC. SAS just left Star to join Skyteam because of AF/KL investment like LATAM leaving oneworld. China Southern left Skyteam because they got closer to AA and haven't joined a new alliance.

FYI: You can still earn and burn on LATAM if you credit to Alaska.
https://www.alaskaair.com/content/mi...partners-latam