It may be an incorrect opinion but I consider Trey Lance a system type quarterback, who did not completely assimilate into the Kyle Shanahan's offensive scheme.
Yes, Lance has been injured but Shanahan was diligent in making him the heir apparent to Jimmy Garoppolo. He witnessed Lance struggle somewhat after finally being named starter in the regular season BEFORE getting injured again. Then saw him struggled again with his system and falling further down the depth chart.
Perhaps Shanahan considered the pros-and-cons of continued investment in Lance? If that is correct, Lance has a new opportunity to (slowly) learn a new offensive scheme, or perhaps better defined as a newly taught one from a new teacher. Maybe Brian Schottenheimer can capitalize on Lance's unrealized potential where Shanahan could not.