Samuel L. Jackson is an actor who really falls in love with some of the characters he portrays. Sure, he is in it for the money, but he really seems like the kind of guy who wants to do things like this (voice his own character instead of someone else). There are articles out there about him and his character of Mace Windu and the affinity he had for playing him.
Anyways, great to hear it will be him voicing the character!