Because Allegri is clueless. Nine players, who almost won the treble last season, looking like headless chickens has nothing to do with missing leadership on the field. Allegri had Conte's team last season and did some fantastic adjustments - respect. But this year, when he has to "start" his own team, he fails miserably. Him going back to the 5-3-2 says it all.
I share your view, it's like what happened in AC Milan. He took over a Carlo squad, he won serie A, then the next season certain champions who had played under Anceletto left the club. AC Milan went on to have a horrible season, absolutely horrible. Allegri had failed to make his own mark on the team, perhaps like now.. in Juventus. Seems he is just "carried" by the work done from coaches before him..
I'm also disappointed that we've taken steps backwards in terms of playingstyle. The 532 should have been dead and burried a long time ago, its weaknesses has been many exposed many many times already. It's a tool which could be used in really tough games were we want to maybe snatch a win or get a draw, but if we really want to be offensive and win it's not the right choice.
He should have stuck more with the 4312 and done teamdevelopment with it, it's the right way to move forward as a team. Instead Allegri is making any sort of consistency and development difficult because of his decisions and choices. For example against Atalanta we played well and won with the 4312, then the next game against Sassoulo, Allegri starts with 532 and the whole game is just a struggle from there. It's just one of many examples of weird Allegri decisions.
He's a good coach if there already is an existing well-drilled team in place, and if there is not, it seems he does not know what to do...
I hope he starts developing a default formation and playing style for us soon.