The reason we have 'struggled' with the Broncos is a scheme matchup issue.
The Broncos are first and foremost a running team.
BB believes in taking away the run first.
Shanahan believes that the best way to attack our defensive scheme is to throw on first down.
When you run a 2gap system, and are playing a running oriented team, your biggest exposure is first down passing.
This is nothing shocking.
The Broncos have had SOME success against our D because they throw against us 1st down, but run enough against everyone else on first down to make BB want to take that away.
The advantage they gain is that we are not equipped to rush the passer strongly from our 2 gap base on 1st down, the disadvantage they create is that if they do not complete the first down pass, they play into our hands.
It has been all about their successful running game creating those opportunites, not that they throw rollout passes.