Hardcourts do a lot of damage to the knees. ........If Roger is ever going to be the best ever he must win the French Open. And in order to prepare for the French Open he must not play Miami. Let the other guys play Miami while he has much more time to train for the French Open.
Hardcourts do damage on the knees to players who can't end points quickly.....or to those who aren't in shape to begin with. On the other hand the clay forces Roger to hit a lot of high backhands which can be wearing on the arm of a one-hander.
Don't forget that lots of people consider Sampras to be the GOAT and he never won a FO. In any case, we know Roger would like to win all 4 in one year if he can. He will do what he feels helps his chances the most.