It is actually still a mystery why Juventus performed so poorly last season. Of course they will miss Ronaldo, but could it really just be that? I mean they've been champions of Italy for the past 10 years, and they were without Ronaldo's presence. Perhaps there was also a bit of underestimation involved. They are favorites for the coming season, but Inter Milan are certainly no less than Juventus at the moment, I think. But anything can happen in the transfer window.
It's normal on a team and not only Juventus, the top teams in some leagues also experience it like in Laliga and the Premier League
If you ask the reason, of course the players involved are one of the factors they are unable to compete with other teams which has cohesiveness
Like Milan, their players are not too special but has cohesiveness when playing this is needed in a team.
Juventus players have certainly seemed to lose their motivation. They are not playing like they want to achieve something. It feels like they don't have a target to chase. To me, it really feels like they are just existing trying to salvage something. The team coordination is not there. The chemistry between the players that we are used to seeing from Juventus players are also not there. And if we compare them with Milan or inter the case only gets worse.