Honestly, it's in part the fans. Hear me out.
Some fans spend so much time pining over guys who blatantly don't want to play here (CP3, D12) and never seriously embrace players who express their interest in Boston. Examples: OJ Mayo has said in the past he'd like to play with Rondo and Josh Smith reportedly listed Boston as one of the teams he'd like to be traded to during that talk last season, and what do fans do? Call them both headcases and turn up their noses. Okay, maybe they might be headcases, but they want to be our headcases. Unfortunately, wanting to play in Boston just doesn't seem to be appreciated.