Lance isn't ready. He's still learning. Rush is way more experienced and has several wins under his belt when had to start.
The team does have wins with him at QB, but he wasn't that good. The defense did a good job of carrying him and when it couldn't against Philly, he played to his level. While the team went 4-1, he completed only 58 percent of his passes, threw for only 5 TDs while throwing 3 ints., and averaged 6.5 yards per pass attempt. Those are very pedestrian numbers and won't win you many games where the defense doesn't lead the charge.
If Dak were injured (season-ending), Lance's numbers might end up worse than that, but you give him every chance to prove if he can be more than a backup, which is what Rush will always be.