Why did Viktor Korchnoi/Kortchnoi leave the USSR?
Maybe the answer could include a little more trivia since he is such an interesting character.
The short story is that the Soviet chess authorities at the time were blocking Korchnoi's progress and severely limiting his opportunities to play, as Karpov had been selected as their favorite son. It was the same sort of impediment that Kasparov later faced when he was on his own path toward dethroning Karpov, and it also echoed the scenario from decades before, when it was rumored/suspected that Paul Keres had been forced to take a backseat to Botvinnik.
While the 1974 match with Karpov was the last straw, it had been building for a decade or more. Korchnoi was never one to just blithely do what he was told, and he felt that Petrosian, especially, was working against him consistently.
In 1963 he was USSR champion, but Petrosian was sent to the Piatigorsky Cup, not him. The Americans sent three tickets, for Keres, Petrosian, and him, but only two were allowed to go.
He complained frequently that Petrosian was being allowed to shake the table in their 73 match (game 5) Korchnoi won, but after the game Petrosian complained that Korchnoi had spoken to him and wanted the match annulled. A very large dispute arose, ending with Petrosian refusing to play, resigning on health grounds. The score at the time was +3=1-1 in favor of Korchnoi.
Korchnoi writes about his early career is "Chess Is My Life" ISBN 06680445280 a slim volume that stops with 1976, right after his defection to the west.