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.
Man oh MAN did that altered title sequence put such a smile on my face I had to watch it 5 times before I deleted it off my DVR. And the fact that the only functional Venture is a midget with one robotic hand is AWESOME.