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.
List is this week's rating, along with the change from the average episode last year, and the average episode so far this year. ShowNetworkDateRat18RLYR%LY18%TYR%TY18%NFL Playoffs (Philadelphia vs. Minnesota) OverrunFOXSun14.29.