The only rational reason i can think, is that they definitely want to finally pass the del piero chapter and do not depend on him anymore, by making space for new and younger strikers. Maybe this not necessarily mean that they have any personal ill will toward him or whatever, but definitely are trying to let him know that they must replace him sooner or later.
In the end of things, i think they dont want to rely on an old 38 years old, as good as he can still be. They want to adventure in finding not necessarily a sucessor (because imo to search for one specifically is just dumb taking in consideration DP´s legacy and all that has to do with him), but to use his spot for another potentially young modern star forward. I think that even the fact that they WILL have to subscribe him for the next CL and use a forward spot is an issue for them (lets get real, Conte doesnt even use him when he really can, now... how many time he will use DP in those super tense european clashes? i find hard to see him performing before the minute 80 sadly.)
In the end, i dont think is anything personal, but they forcefully just want to turn the Del Piero page. Of course, im aware that del piero wanted to go on, and this is not ending in del piero´s term, but in one way i think he is getting the idea. After all, he is definitely not in Conte´s plans, as he is being used like some kind of "memento" or "token" that comes in and does his magic, like in the old glory days. Sometimes i feel that he is almost used for a "nostalgia" thing instead of true footballing technical reasons.
We all saw him play this season and how good he still is. He definitely rates himself still high and competitive ( for a reason) but i think he also understands from were they are coming. Because of this i think he is not creating a soap opera out of this and just taking it like the champ he is.
Personally, i just want to see him retire in this club, as i dont care how many good players can come....
A del piero, a platini, a boniperti....those kind of players come once every 30 year.....and in the case of del piero, a champion not only on the field, but a true captain, a gentleman and the most charismatic footbller i have ever seen...... i dont really see another one like him coming in at least 100 years