Here is why:
During the 2014 season, the Cavs were a lottery team and they struck a gold by hitting the #1 pick. Lebron knew that the team is capable of surrounding him with talent with those assets they have. They may have failed in their first try but they won it all, a season later.
This Celtic team is already better place to be than that Cavs roster. Not only the team won 48 games without superstar on the team, they have tons of cap space and lottery picks to spend. #1 pick help the Cavs land Love (although he wasn't much of an impact).
We already have a Kyrie in Isaiah. Smart and Bradley could be our better version of Shumpert and Smith. They have Tristan Thompson as a rim protector but with the cap space we have, we can either sign an impact big via free agency or use our assets and cash it through trade.
Kevin Durant already stated that he wants a great coaching staff and CBS is one of the rising coaches in the league. He also stated the he likes the fanbase in Boston. Danny and Durant's mom have some connections too. Not to mention Perkins is the one suggesting that Durant should sign with the Celtics.
People say that he should sign with the Warriors and the Spurs but no. It will only hurt his legacy and people will hate him worse than Lebron with "The Decision, Durant Edition". Outside OKC, Celtics have the biggest odds on landing Kevin Durant.