Probably because if Boston moves Rondo or Pierce they'd have room to bring in another all-star along with him.
Ding, ding, ding! People thinking Ainge would be asking him to come here to play with a bunch of old dudes don't seem to realize the limitless flexbility Ainge has this off-season. They essentially have no guaranteed money on the books for next year as Rondo could easily be dealt to a team under the cap and Pierce could easily be amnestied (as much as it would suck to do it), if that's what Dwight wants.
The Celtics can allow Dwight and Deron play together like they want to and offer a far better supporting cast than the Nets can. Just one example: you could probably deal Rondo to the Sacramento Kings for Demarcus Cousins who is very cheap ($3.8M) next season and keep Pierce, if Deron and Dwight take a less than $2M per year discount from the max (Miami Heat style). Or if they aren't willing to do that, you amnesty Pierce and have roughly $13M available to spend on guys like KG/Ray/Jeff Green or maybe Gerald Wallace.
It's still a long shot because who knows whether Dwight even reaches FA (could see a Chris Paul esque opt in to secure a trade) and he may prefer living in a place like Brooklyn or LA over Boston even if we can offer him a significantly better roster but if Dwight reaches FA, Ainge can make a hell of a sales pitch and it's not "come here and play with a bunch of old dudes". It's called having a shitload of cap space and a significant trade asset in Rondo.