Look at the conservative stable.
Boris Johnson is the best the Tories have. Who would you have it be? Raab? The man's already backstabbing others jockeying for position.
Nigel isn't a Tory, and that would require a general election that would put conservative leadership at hazard.
Let him be the voice of Brexit. No. 10 needs someone who's not afraid to sever ties with the EU entirely, which Johnson would see to by October.