Why would Milan want to sell De Jong?
It's not necessarily Milan/Allegri who want him out. I expected rumors about him being on his way out of the club because:
(speculations)
Nigel's POV:
things havent been the way Nigel thought it would be even though he expected a transitional year. He most likely didnt expect the team to struggle this badly, the squad to be this shitty, to play most games in dirty, empty stadiums, to be considered a flop signing by media and blamed for our bad results even though he has been good and he doesnt seem to have a good relationship with Allegri. Like I wrote earlier, he seemed angry and ignored Allegri when he tried to shake his hand. Him being benched or "rested" the whole game against Palermo was obviously not his own choice. One of the reasons he left City was to play more.
Also, it's pretty clear that one or the main reason he chose Milan was because of his friends, Boateng and Urby(and also Sneijder). I dont think he likes the way Boateng has been treated, or doesnt want to continue in Milan if Boateng leaves the club. Urby not starting that many games may also have made him dislike Allegri even more.
As for Milan's POV, they obviously bought him to be the lone anchor in front of the defense, but a double pivot system suits him much better, which Allegri seem to not like. He's also on pretty high wage(3m net) and the management being the idiots they are probably expected him to make a huge impact on the team, despite him obviously not being the playmaker we needed badly.
If this were to be true, then I blame bad research from both parts and a huge facepalm to the club.