Juve wants fresh blood, Juve is aiming (in sporting sense) at Bayern, in financial sense at ManU. They want younger manager. Plus, allegedly, he was against Ronaldo's transfer as it wasn't financially sustainable unless they win two CL in a row cause it's 100 mln amortised for number of years of contract plus 60mln euro gross salary every year which is huge for a club with 40k stadium capacity and the title sponsor paying 25+ mln a year (compare with the likes of Rakuten paying 60-80 or whatever they pay to Barca).
I also heard that he was more faithful to John Elkann. In the internal fight between two relatives for influence and power in Juve (I think Agnelli is second uncle to John Elkann, IIRC. Or probably there isn't such an expression as "second uncle" in English, I guess it should be "cousin once removed) Elkann is the main guy in EXOR and heir of Agnelli family, but Andrea wants to be a top dog at Juve. So maybe that's also the case of Marotta being removed from managerial position.
They also think about executing that final leap to the Europe's absolute best, so 60+years old manager is viewed as redundant and obsolete and his skills at transfer market aren't important now, after all, Real or Barca don't need Bosman transfers to be successful, they need transfers of players who can add that missing piece in puzzle, they are willing to pay, not save costs.
For us, though, his skills will be indispensable (in this transition period). He never splashed 30+ million on useless players (like Joao Modric and Gabigol, Kondogbia to the lesser extent). Costa, Pjanic,Alex Sandro, Dybala were all very successful transfers, his Bosman transfers were awesome too.
When we will be (and we will, I promise u guys) much stronger financially with the new stadium and will afford purchasing absolute superstars of nowadays' CR7 statute Marotta won't be needed. But for now if he will pull off same stuff he pulled off before all these years we can become second force in Italy in a year, maximum 2. We have a core of the team now, we need a few players aged 24-27 to become backbone of a team for 4-5 years, those new Sandros and Dybalas. Not useless pricks like Candreva, JM and so on. Hope he can help us lure those kinds of players.
What will also help is that we are much stronger financially than Juve of 2010/11. He had to buy Vucinics and Matris for less than 20mln cause they couldn't buy Aguero for 50. At least us have strong support from Chinese sponsors which ensures that we don't need to buy fodder type of players for 15 mln just for the sake of quantity.