Because two people near the top of the conspiracy, with broad knowledge of its details and where the supporting evidence is, have decided to fully cooperate.
And because SBF likes to talk and told his (original) lawyers to, as he tells it, “go fuck themselves” when they advised him to stop publicly confessing his crimes.
In the modern era of email / slack / whatever - every smoking gun piece of evidence is a ctrl-f away. SBF is publicly testifying and pointing to where the bodies are buried, FTX the company is cooperating and co-conspirators are cooperating.
Half of keeping your nose clean and keeping people accountable is cornering them into reading/writing something. This is a great way to make yourself nobody's friend, but also a good weapon for halting something by refusing to do it without getting it in writing first.
"In June 2018, Theranos announced that Holmes was stepping down as CEO. On the same day, the Department of Justice announced that a federal grand jury had charged Holmes, along with Balwani, with nine counts of wire fraud and two counts of conspiracy to commit wire fraud." [0]
[0] https://www.businessinsider.com/theranos-founder-ceo-elizabe...
Other crimes are very easy: misleading (prospective) investors just requires that you fail to disclose something. No need to prove intent, even something very small will do. And as long as there is at least some connection to the US, US courts will hear the case.
Prosecuting SBF for defrauding customers would take years. But getting him for failure to disclose to investors (which seems to be what they're after) is really easy. So they're powering through.
They also now have 2 co-operating witnesses.