I agree that one reason is definitely harassment by the OTT Stannies who were Johnlockers and/or disappointed with the direction Sherlock took. What PR is trying to brush under the carpet, however, is just how badly BC’s fauxmance related BS wrecked the working relationship on Setlock:

198. ENTERTAINMENT LAWYER 01/18 **#2**
No amount of money is going to get this foreign born actor who has been in some of the biggest movies of all time to go back to this foreign television show. He just doesn’t like who this foreign born dual threat A list actor has become. Martin Freeman/Benedict Cumberbatch (“Sherlock”) (Revealed by Enty on 1/22/2017)

231. ENTERTAINMENT LAWYER 09/21 **#11**
Another little nugget to add to the feud of these two actors from a BBC show all of you know. The slightly more famous of the two disliked his co-star so much that when he was required to give a line for the lesser known actor to respond all he would say is, “Blah blah blah, your line.” It drove the other actor crazy because he had nothing to play off at all. Benedict Cumberbatch/Martin Freeman (“Sherlock”) (revealed by Enty on 9/29/2017)

BBC shows won’t be the only thing BC’ll lose if this goes on…