The first SLS flights will use available Block II RS-25D engines left over from the shuttle program, and when those run out (and if SLS is still flying) the rocket will switch over to the RS-25E, a cheaper, expendable version.

The F-1 engines used on the first stage of the Saturn V were built to burn Kerosene, and the RS-25 series has no common heritage.

It boils at a lower point than oxygen, so you have to insulate the tanks from each other, it's very.. undense (sparse?) per volume so you need bigger tanks, and it's the smallest molecule that exists and makes leaks and shipping harder.

Methane is slightly less efficient but way easier logistically.