His friends have stated that Warren is a very hands-off person and does not like to get involve with running a foundation and deciding who gets money. He likes to hire other people to invest his money !!!!
He'd have to man his own foundation and go through a lot of personal problems getting it going, plus it might be too close to his children who he is not leaving the bulk of funds to.
With the Gates foundation, he likes what they do and they are already set up.
IMO they do what he would want to do and he avoids family problems, lawyer fees for no good and his own precious time.