Rattler seems like the preferred choice of a coach on his last leg. If it works out, the coach gets credit for bypassing the #2 because of the vision he had for the #3. If it doesn't, he can say the rookie "needs more time to develop" as he did after three losses. He probably didn't think he could win the games anyway, so why not pin it on the rookie QB who almost certainly didn't have a chance--whether we like it or not, he was a fifth round pick for a reason.
Also notice that Rattler, with nearly three full games of extra experience in the offense, was back to the #3 after Carr's return. They thought a guy who only has a couple of drives of regular season action in "garbage time" was the better backup in a game that Allen likely knew he had to win.
Unfortunately for Allen, seven losses in a row were too much for him to blame it on injuries/rookies/etc.