A little history about the NBA free agent process. The first unrestricted free agent in NBA history wasn’t until 1988, and that was Tom Chambers, so at the most there has only be 22 possible years to sign free agents, not 30+.
From ’88-’93 the C’s were an established team with Bird, McHale, Parish, Lewis, etc. No real room to sign free agents.
From ’93-’95 were there any Tier 1 free agents available? The Celtics didn’t do too bad though signing Xavier McDaniel, Blue Edwards, Pervis Ellison, Domnique Wilkins, Dana Barros (coming off an All-Star season), etc.
Big Free agent class in ’96 with Shaq, Dikembe Mutombo, Allan Houston, Mark Price, Dan Majerle, Reggie Miller, Chris Childs, Juwan Howard, etc. The salary cap is only $24.4M, but the C’s have $25.8M in salaries. No cap room to sign any free agents.
From ’97-’99, The C’s are rebuilding and don’t have cap room. Best free agents that switched teams seem to be more experienced guys (Pippen) who wouldn’t want to come to a rebuilding team, and young guys like Antonio McDyess, where we already are committed to Walker at his position, plus we were already over the cap (in ’99-‘00 we had about $46M in salaries and the cap was $34M).
A big class in ’00 with Grant Hill, Tracy McGrady, and Duncan all up for grabs. Duncan stays put, it doesn’t matter because we don’t have any cap room anyways (in ’00-’01 we had $51M in salaries with a $35.5M cap.) Chris Webber is a free agent in ’01, but resigns with the Kings, don’t have the cap room and already have an All-Star power forward.
From ’02-’05 we’re a capped out playoff team. No room to sign free agents. Besides, what significant free agent switched teams besides Carlos Boozer, who plays the same position as Walker to whom we’re still committed? Steve Nash would have been a nice signing, but again we have no cap room. Kobe Bryant was a free agent, but did anybody really think he was leaving LA?
’06-’07, we’re a capped out sinking ship, who could we even sign?
’08-’10, we’re a capped out championship team, that a lot of guys want to play for (Sheed, O’Neal Brothers sign, PJ Brown comes out of retirement, Cassell and Marbury get bought out and come here, Corey Maggette even talked about taking the mid level to sign here). If we had cap room, we probably could have signed one of the major free agents over the past few years: Baron Davis, Elton Brand, Boozer, Amare, Joe Johnson, Rudy Gay, Maggette, Ben Gordon, Charlie Villanueva, Josh Childress, etc.).
So basically, I think it comes down to, there have only been a few major free agent classes in NBA History (’96, ’00, ’10 in my opinion, let me know if I’m missing something), and all those years we didn’t have cap room, which can be chalked up to bad management (except for our recent run). Has there been a year where we had cap room where we came up empty like New Jersey did this year. If anything it seems like the Knicks have continually failed. Didn’t they try to land Webber in ’01, Rasheed in ’04, and everybody in ’10? They’re the big market team, with the night life and the glamour that Boston doesn’t have, but it doesn’t seem like the free agents are signing there either.
(using wikipedia for salary caps and basketball reference for team salaries for that season which doesn't show how much off season cap space they have, so my numbers may be not be completely accurate)