To be fair to the SDNY, it's only been about 4 months or so since they kicked their Giuliani investigations into high gear.
Bill's just distressed that people don't like him and feels this is a momentous opportunity to put himself in a higher category of regard than such past mayoral luminaries as Giuliani and Bloomberg.
Actually use to work for the mayors office and can add some context and Tea...
1) Bernie was the celebrity speaker at Bill's final inauguration. They have a legit relationship, de Blasio was always going to endorse him unless there was a clear front runner ahead of him and knowing Bill he probably would still endorse Bernie.
2) Bill and his people
KNOW everyone thinks he is an asshole. A lot of Bill's run was he was desperate. He should have gone after Cuomo or Schumer, but both of those moves honestly would have probably led to L's and made it hard to ever do anything else in NYC either.
3) Bill is in find a new job mode. Besides trying to get his wife the Brooklyn borough president gig (aka He endorses Adam and then Adam endorses Chirlane). The next move would be a secretary job for himself in Washington. Potentially education because he would have a history of success with Pre-K and 3k and while his push for community schools mostly failed, between them, charters, and everything else. Bill would be uniquely positioned to have both a plan, a record of success, and staff to do some interesting stuff nationally. Forget that most the heavy lifting on education in NYC was done by Deputy Mayor Buery. That's the plan. If Bernie gets elected I give the plan a good 75% chance of working too, because de Blasio really did help the schools.