BSN was purchased by Glanbia, who also own Optimum Nutrition and American Body Building Products. Once they took charge in March, they started to make some changes. Not much, but they did not renew Bob's contract. It happens all the time in various companies out there, with many competitors and spokespeople.
Don't worry, you will see Bob a lot, as he is the 2011 Mr. Olympia emcee, as well as the FIBO Chicago Pro emcee, as well as quite a few others.