Because his usefulness as a prop for photo ops had come to an end. The last time I saw a photo of Buddy with Bubba, Buddy had just knocked him on his a** in the driveway of the house. I'm surprised Buddy lasted this long after that incident.
And if you remember the Big Creep made a hyper-sensitive comment about how the press got a good photo ... that Buddy had never knocked him down. Clinton never did anything to deserve a dog ...