The split was over the failure of “Roots”, and its followers. One of them I forget who, wanted to go country, like Jerry Lee Lewis at the time, and the other (probably Don, but I’m not sure) wanted to stay in rock.
After they got back together with the Paul McCartney song written for them, they stayed together but Salman Rushdie, who knew Don, reported at one time that they still didn’t get along and didn’t speak to each other off stage. I wonder if they ever reconciled.
Whatever the reason... I hope he finds peace with the Lord.
The split was in part because of Don’s drinking and drug use (ironically created by Phil’s first wife who got Don hooked), and you are right Don wanted to go Country (and did for his solo albums) while Phil went more pop (for his solo albums).
The song McCartney wrote was “Wings of A Nightingale” (great video too) off their EB84 album.
I am a walking encyclopedia of Everly info. LOL