>>149541017
They were both Perfect Earths.
On Earth-15, Zod was Superman and they had beaten enough of their bad guys to pass on the mantle to their sidekicks. That's the Earth Jason-Bats is from. Superman Prime (an aged up Superboy Prime) blew it up having one of his "not muh Errf!" temper tantrums.

On Earth-51, Batman killed the Joker after Death in the Family and took out all supervillains to the point to where superheroes could reveal their secret identities and make the world perfect.
Until Superman Prime punched Monarch so hard it nearly destroyed that universe.
Then it got better.
Then Karate Kid brought a future virus that turned the whole planet into Jack Kirby's Kamandi series.
Then it became Limbo.
Then it became New Genesis.
Then it became a combination of New Genesis and Kamandi.