For the whole board of the Social Mobility Commission to resign with its chairman, Alan Milburn, condemning the Prime Minister’s commitment to the agenda is pretty damaging. But this attack was inevitable, for reasons that haven't (so far) been picked up by the newspapers.

Ever since Theresa May took office, she has shown almost no interest in the Social Mobility Commission, set up under the coalition years. No10's approach seems to have been one of strategic neglect. Alan Milburn's five-year term came up for renewal last July: Justine Greening, the minister responsible, was keen for him to stay. But No10 refused, and asked her to come up with other names. She refused (No10's authority is shot, so Cabinet members do engage in such acts of defiance) and Milburn stayed on in a weird limbo. No10 made clear that it didn’t want him, but couldn’t decide who it did want instead.

This fitted a trend. No 10 had blocked a whole load of high-quality people from joining the Social Mobility Commission's board. Often you’d get dozens of people of people apply to be commissioners, a candidate would be nominated after a long procedure and then No 10 would just veto. So out of the initial ten commissioners, just four remained. It looked like No 10 was trying to run the commission down.

So Milburn quit. Then the vice chair, former Tory Cabinet member Gillian Shepherd, also quit in protest over Milburn's shabby treatment. In such circumstances, the remaining (two) SMC board members didn’t have much of a choice but to resign.

Yes, things are busy with Brexit and investigations over Damian Green’s browsing history, but someone in No 10 should have seen this coming. The Social Mobility Commission had been weakened, disempowered and chronically understaffed - it looked as if it was being starved to death by No10, rather than just wound down. But why?

Team May always saw Cameron's focus on the poorest as a kind of de haut en bas way of looking at society. Like so many of May’s ideas, this has its roots in a desire to define herself against David Cameron. She thought his social agenda reflected an Etonian view of the world: the rich man in his castle, a poor man at his gate. So she scrapped Cameron’s almost-completed Life Chances strategy and supplanted it with her own: the 'just about managing', a focus not on those at the bottom but a wee bit up from the bottom, whom she felt had been neglected. She had a point. But help for the "just about managing" turned out to be more of a soundbite than an agenda.

We had her grammar schools, but that agenda was pursued without any evidence that academically selective education helps social mobility. Indeed, bringing back grammars looked like another way of of defining herself against the privately-educated Cameroons.

So we end up with a position where Milburn knew he was for the high jump, his reappointment having been blocked by No 10 and his Commission deprived of people and resources. So if you’re Milburn, do you walk away quietly – or loudly, seeking to make a final, headline-grabbing point about a Tory government’s commitment to social justice? It’s not a difficult decision. It's worth underlining that this was Milburn's attack; the other commissioners have said nothing.

Much of Milburn’s j'accuse, in my view, is unfair: this government is doing plenty to help, though by enacting policies of the coalition years rather than its own. But No 10 is guilty of losing interest in this hugely important agenda, discarding what could have been a powerful means of demonstrating the government’s wider purpose. Milburn today asks if it really does have a wider purpose, beyond Brexit. It's a depressingly good question.