He is doing a very bad job tactically and in fitness preparation, so its not surprising he is not popular with most recent group of former players.
But also even when successful for the past decade, he never struck me as a coach who has player management skills as a priority. He was openly disliked or semi conflict with several Milan players/veterans. Rubbed some Juve players the wrong way too even while winning 11 titles in 5 years etc. Personable and open communication channels or handholding type of leadership not his forte.
Its a combination of lacking that ingratiating touch with them (as several top coaches are known for), and as Montolivo wrote it well, his lack of preparing or instructing you what to do in general play/attack gameplan tactically, not being too popular with some players. Who prefer to have clear and definitive orders out there. Instead of too much confusing freedom.