Why?
His seat disappears due to constituency boundary changes and he has decided not to contest the adjacent seats against their current selected candidates.
https://www.bbc.co.uk/news/uk-politics-66045931
He has said he is willing to remain in post until the election, but, as he is standing down, will be replaced at the next reshuffle anyway.
https://www.bbc.co.uk/news/uk-66213245
You can’t contest a general election without knowing who will be Defence minister if you win…