The reason is because he wants to spend tons of money and he wants to spend on players he wants, not who management wants. Isn't it obvious by now that he got tired of having little input on the overall direction of the team? We were going to buy Morata, Coman, Evra, Pereyra whether he liked it or not, so he quit.
He can have his Pelusos and Padoins because they cost nothing but he had no say in the bigger investments IMO.