This is one area where being a fairly balanced team puts the C's at a disadvantage. There are very few free agents who could supplant a starter at the 1-4 for the C's, but no C's starter that is really top 3 at their position. Therefore, basically every quasi-starting caliber free agent is better off signing for a different team, usually one that has a legitimate top 3 player at at least one position at the expense of starting borderline players at other positions, where a ring-chasing vet could just jump in and play major minutes. Teams built around a couple elite players with other roster holes are more enticing for those holes to be filled. For example, Miami will attract every elite ring chasing PG and C;, LA should attract ever ring chasing SF and PG, but Boston is really only in a position to chase ring chasing C's, but so is everyone. Chicago is more attractive for a Vet SG. Etc.
A motivated Baron is basically as good a player as Rondo; no way he comes to boston to play behind rondo when he could start on a very good team elsewhere.