It takes time to put together a case and when the case is against someone as high profile and with the resources available to them as and ex POTUS, that means you gotta be very damn sure your evidence is good and your charges will stick.

It's not like they caught him speeding on the M5. He's accused of things that require hard evidence and proof. I'm sure they spent a couple months just playing out different scenarios of charging him with various crimes until they found a set that seemed foolproof.

The only problem with foolproof is that people underestimate the ingenuity of fools.