Yeah, its actually very easy to see why, so i'll enlighten for those who have $$ in their eyes and want the sale.
First of all,for the money of a sale, we cannot get a player(s) that gives anything close to the impact vidal did.
People will say "2001 zidane" but thats quite wrong. Wages werent the issue now that they were back then, Serie a isnt as attractive as it was and it doesnt have as good players as back then.
The Martinez+Isco is a good example. Two players from a different league,playing teams that currently are better then juventus with a bigger reputation.
Do they even want to come here ? On a wage we can afford ?
Vidal wont get an offer much over 50mil. Bayern paid 40something mil for Martinez. They'll want to recuperate most of that. Isco wont be wanting to leave real. He'll need to be convinced to come to a team that just sold its biggest player ? And madrid will ask a stupid amount of cash.
In the past, juventus was a huge name. Players and agents would leverage a deal with their club to come here. Right now thats not happening, and thats a big difference.
We cant sign anyone that has Vidals impact. So we wont sell him, because it will be a big loss for us.
Added to that, if juventus get brandished the "seller team" mark like jukazem said, is making it far worse. Players will think even lower of us, and the likes of Pogba and Coman if he breaks trough, will want to move cause juventus isnt seen anymore as the final destination, but a trainstation to the next stop.