I think the answer as to why Bergeron is not signed is the same as most of the remaining guys that would actually command money, cap space. Boston does not have much free space. They need to move someone to make room for him.
Yes, teams can go 10% over, but then they really are over a barrel. especially when the guy is not going anywhere else, stay cap compliant and work on your side trade to clear space.
This same thing is likely what is holding up a Kadri signing. He must have 1-2 sure fire landing spots, but is waiting for a cap move. Both Colorado and the NYI could, as examples, easily clear enough cap space but they would be making moves like Columbus and selling something decent for nothing — or trying to move a bad contract for a big price. By holding the signing, they at least are under a bit less visual pressure and might get a better deal.