I can understand passing on a 35-year-old who has had microfracture surgery on both knees and whose Chinese basketball team wanted to outright cut him if their league hadn't pressured the team, leading to that weird buyout and refusal to grant a letter of clearance.
So, he was basically an old, injury-prone big coming off a season where he was cut from a Chinese team in all but name and proceeded to shoot 44.1% from the field and 37.0% from the free throw line with a 11.5% total rebounding percentage (or pretty much rebounding exactly what Brandon Bass has been doing the past two seasons). He reportedly tried to start a locker room revolt against Vinny del Negro during his time with the Clippers.
Kenyon Martin was averaging 14 points and 7 rebounds per game in a league where DJ White was averaging 21 and 10 and Shavlik Randoloph 32 and 14.
Given all that, he sounds like a huge ball of risk that you can't blame a team for not being interested in.
He was also said to have been insisting on more than the minimum salary when the season started. In Ainge's place, I probably would have asked him to sign a minimum, partially guaranteed contract.