A few reasons. One is that the Nets are resigning their own free agent and can exceed the cap to do so. Another is that Brooklyn is in a similar situation as Denver with Faried a few years back. Don't really want to overpay but can't afford to lose talent for nothing. It's a bad deal and a mistake IMO but they are stuck between a rock and a hard place.
Portland can exceed the cap for Matthews but every other team would have to have the cap room or do a sign-and-trade to give him a $15M contract. The Lakers have that kind of room. So does Philly. Beyond that I'm not sure.
Also Mavs, who are believed to be the leaders in Matthews' sweepstakes. Parsons, DAJ and Matthews were all hanging out together last week.
as far as nbawowy.com database goes
1/1/12 - 12/19/14
Celtics w/Rondo - 6108 poss, 102.7 PPP for - 106.7 PPP against; Jared Sullinger - .488TS%
Celtics w/o Rondo - 11861 poss, 104.0 PPP for - 105.5 PPP against; Jared Sullinger - .519TS%
(Jared Sullinger was the only guy on their roster during this time, that resembles Cousins with the amount of offense he creates for himself)
Cousins saw barely any spike in efficiency, when Dre entered the game replacing Ray late in the season, while Rudy looked much better with Dre than with Ray. Gay is vastly improved, when he receives the ball on the move rather than in stationary position, so I can see why Rudy would push for Rondo besides them being friends.