The top answer says:
“the Chief [Brexit] Negotiator.. then resigned when [he was] unable to
deliver anything that [he] personally approved of as a consequence of
What happened was that the Prime Minister had two Brexit strategies running in parallel. One in public, led by the elected representative Brexit Secretary (David Davis), and one quietly behind the scenes led by a civil servant named Olly Robbins.
Brexit can be split into two parts: the Withdrawal Agreement and the Future Relationship.
The Withdrawal Agreement is the legal agreement defining the process by which the UK will leave: for example financial settlements, dates and rules for any period of transition.
The Future Relationship is the legal text used to define the UK/EU relationship after any transition period. ie. for the long-term.
As Brexit Secretary, David Davis was unhappy with elements of the Withdrawal Agreement as it slowly emerged in 2017/early 2018, but his team’s primary focus in early 2018 was the really important text: the Future Relationship.
The Prime Minister then announced that the Draft Future Relationship text would be revealed on 12th July 2018 at her official country house named Chequers.
It immediately became clear to David Davis, the Brexit Secretary, and the people’s representative in this area of policy, that his work was to be ignored and that another document, written by the second team inside the Civil Service, was to be used as the basis for Brexit. Davis then resigned (along with his deputy).
So the Brexit Secretary’s manager revealed that she had gone behind his back for a period of months to develop her own plan to replace his. This was a firing, not a resignation.
The answer to the wider question as to “what happened to all the Brexiters?” is simple: Brexit was a revolt against the political establishment. So there were almost no Brexiters in a position of power in the first place.
There were a handful (a minority) of Brexiters in the Prime Minister’s cabinet, but they have mostly resigned by now in protest at May’s deal.
And Nigel Farage was an MEP, not an MP.
You ask about Boris in a comment. Boris did launch a campaign to run for PM, but shortly after thereof another MP (Michael Gove) who was also reputed to be a Leaver stepped up to be in the running for the leadership. Gove was widely perceived to be the more serious, more capable and more rounded candidate and therefore likely to win. Furthermore, Gove immediately went on the offensive against Boris who had been a close ally only hours earlier, saying he “cannot provide the leadership or build the team for the task ahead.” This sequence caused people to say that he had “knifed Boris.”
This forced Boris to re-evaluate his chances of winning, and he dropped out of the race. Gove then subsequently lost to Leadsom. Presumably this loss was contributed to by his perceived poor conduct wrt Boris. His motivation for running in this way was unclear. However, with Gove subsequently backing both the loathed Withdrawal Agreement and a Future Relationship scoffed at by Leavers, questions can be raised about his true status as a Leaver. If Gove was actually a Remainer, then this would not be without precedent. Philip Hammond (the current Chancellor) indicated prior to the vote that he could vote to leave the EU. It has become abundantly clear since that he is a firm Remainer, going so far as to call the Leavers in his party "extremists".
Leadsom dropped out after a media offensive against her for (benign) remarks about Theresa May’s childlessness. Her inability to form a coherent response to this relatively mild media spotlight, caused her support to waiver and undermined her Prime Minsterial image, although by this point she merely needed to win the membership, who could well have backed her for her stance on Brexit alone. I don’t know why she withdrew. Perhaps she simply couldn’t take it, perhaps she was an intentional spoiler candidate.
For reference, here are some relevant paragraphs from a Huffington Post article on events. All the big news outlets had a consistent description of events.
The most important paragraphs say:
Davis made clear to the PM last Monday that he was “not happy” at
leaks that she was to revive the soft Brexit option. Incredibly, he
first found out about the details of the new scheme on Tuesday
afternoon, after the BBC had been briefed on them for that morning’s
“There’s an important constitutional point here,” one ally said. “An
unelected civil servant has been used to bypass the party, Parliament
and the Cabinet strategy and negotiations committee. The Cabinet
Office ‘Europe Unit’ seems to be running the show. Even Tony Blair at
least paid lip service to Cabinet Government.
The source continued: “We had a situation where the Foreign Secretary
was only given a big thick folder on the Brexit white paper on 2pm,
the day before the Chequers meeting to discuss it.”