John Stewart announced his retirement after 17 years of doing the Daily Show. 17 years! It hardly seems possible. But while watching Morning Joe on MSNBC this morning one of the panel said this: "It may be John is doing this as a favor to Brian Williams." To take the attention off Williams and his being suspended for exaggerating his experiences while reporting news. He gives his friend a break from being the center of none stop attention.
I highly doubt this is why Stewart decided to end his show's run.