There are at least a few theories as to why Juventus didn't sack Allegri up until now, but I really don't care about the theories because Juventus are so screwed right now and Allegri can't fix this. Because so far, when the big teams get bad results that keep repeating and will even be eliminated from the Champions League. So the team would have rushed or planned to find a new coach, but Juventus didn't do that and it looks like they are fine with such a dilapidated condition.
I think it's not really a theory at this point, the outright truth is that they do not have any other alternative they can hire to make the team better so the only thing they can do is keep him, or just bust it open and lose the season with some interim coach and focus on next season instead. Right now is too early to think about next season so it's not going to really look like it will make any sense to fire him. If he keeps going like this for another 10 games or so, then they may think about it, or by the end of the first half the season if they are still bad then they can fire him, but it is way too early to give up on the season for now.