Brandon Marshall is all about himself.
He is a locker room cancer who performs selfish and inexplicable acts like calling a press conference for himself to discuss his BPD (for no reason other than to sake his own vanity), instigate locker room ruckuses like the one after the loss to the Bills at Soldier Field last fall, challenge fans who troll him on Twitter to boxing matches, and of course devote valuable time during the regular season to carving out a media gig on Showtime.
All in all, it is pretty clear that, just from last season alone, he is not fit the profile of a player who would flourish in New England.
Be that as it may, he did this under the Marc Trestman/Phil Emery regime, which might explain a fair portion of it...