Because Shanahan the douchebag always plays extra conservative against the Rams so they will not beat themselves. They'll run the ball and take short passes. In our game against them, I don't think Garapolo threw a pass longer than 10-12 yards the entire game.
And then our defense kept playing soft and off the ball so the whiners didn't have to deal with the pass rush. Jimmy G string just threw the ball to spots which were made easy when Kittle and Deebo Samuel get free releases and the DB is 5-6 yards off them.
Then we also played into their hands by turning the ball over twice. That's been Shanny's game plan for the last 3 years.
If we want to beat them we have to play a little smash mouth ball and don't worry about having to punt the ball away. We just can't turn it over. And yes, starting fast and scoring first would also help. I don't recall us having even had a lead at all in the last several games against them.