The lion's share of the 4 year investigation involved lengthy court battles, fighting over Trump's financial and tax records.
The Manhattan DA's Office has had those records for a few months after finally prevailing.
What they have charged thus far is criminal, but certainly not typically charged, at least without allowing a civil settlement to square up the unpaid tax bill.
If they have nothing else to bring and/or are relying upon cooperation to move the ball forward, then I do not understand why they would have charged as they did.
But, there has always been a reason why Trump would never agree to release his tax/financial records to the voters or prosecutors. Trump obstructs at every level. He is a dishonest corrupt person. Whether anything becomes "criminal" only happens when 12 jurors say so.