Because Stafford can't play the RPO.
Niners were really worried about containing Hurts' runs.
They adjusted their pash rush to hold the end vs having folks like Bosa speed rush and leave wide gaps open.
When Eagles overloaded a side, Niners would overload on the strong side to box Hurts in.
This however, left a 2 on 2 matchup at the line with the offense having numbers if they pulled.
Hurts kept handing the ball off instead of keeping which then led to some decent runs by their RB.
Remember that replay that showed Warner just stuck there reading Hurts. They REALLY keyed in on him. So what do you do, you take Warner out of the equation and hand the ball off.
Niners did adjust for this in the second half.. which then the Eagles adjusted where you saw the opposite happening with Hurts holding onto the ball on the option. Niners played a bit more balanced in that formation and keyed in more on the RB.
Cat and mouse game which the Rams can't play unless they want Stafford to go to early retirement.
