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.
I preordered the MST set a month or so ago and then totally forgot about it until this past weekend. I think it's arriving Wed or Thurs. Can't wait...it's pretty much the only thing I've gotten myself for 2008.