I don't think he is called that for any particular reason. His fellow politicians viewed him as a goofy dumb Cowboy so they called him "Bingo Bob." If he did earn that nickname in some fashion the writers have yet to reveal it.
The episode where he is first mentioned only make reference to the fact that he is perceived to be in the pocket of mining interests and is not particularly swift on the uptake. But no actual source of the "Bingo Bob" moniker.
Talking Dead alluded to who these guys might be. Its Glenn at the end, right? The couple and their kid sister were shady all the way around. Daryl has perhaps gotten too pussy whipped by everyone. That was the message of the story.