There are various reasons for this including:
1. Certain clocks start upon issuing the counts related to timeliness in the legal world.
2. Most often there is still investigation going on but they want to get the person locked down immediately and they can continue adding counts later
3. Sometimes prosecutors will keep additional counts available as a way of cutting through defense stuff. Oh you want to play games... well I have these additional counts I’m going to add now. Or to get cooperation to take down other bad guys.
4. Sometimes a count will fall through due to a dead witness, paperwork issues, etc so they will take a count that is just as bad and add it in as an additional charge at a later date.
5. Sometimes they just don’t want to overwhelm the jury with 200 counts and the time it would take to present all 200 in court will present 5 as in the end the outcome will basically be the same. I can’t imagine the horrors some of the grand jury have already been subjected to - and now the trial jury will need to be.
6. Each count likely has lesser included charges attached to each one.
Just some stuff off the top of my head.