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.
This episode didn't feel as good as the last few weeks, but it definitely had its moments. I thought for sure "Black Jesus" would get at least one or two more mentions, but the one cutaway we got was great.