The curse of sin is death. The fact that most of the inhabitants of the earth have rejected the curse-breaker, Christ Jesus, leaves the earth under the curse.

Outside of Christ there is desolation and misery.

For rejecting the Creator and Saviour Jesus Christ the earth will be scortched to eradicate the spirit of rebellion.

The few that are left are those who have never heard the Gospel of Jesus Christ and so are allowed to live in the MillennialKingdom as humans.