Why the holdup? It has become political.
The big question is if Boeing was not forthcoming about the MCAS and the FAA did not know, did not care, or just missed it, is what else might be lurking in the MAX that may also have unintended consequences.
There are lots of new systems on the MAX to make it fly better. If the MCAS (which was there for a similar reason) can screw up and lead to a crash, what about the other new stuff?
For instance, there is a new FBW spoiler system. One of the things this does is deploy the spoilers a bit on approach at flap 30 or 40 so the flight spoilers will raise slightly to reduce lift necessitating a higher AOA and hence nose attitude to give an "acceptable nose gear contact margin" (because the wing and landing gear geometry has changed from previous 737 models). I'm not saying it could occur, but what happens if this system puts the spoilers full out, or full out on only one wing? Was this tested? How extensively? Does anyone actually know? Boeing has already shown that they may not have thought things out all the way through. Needless to say, a full spoiler deployment at low altitude at approach speeds is unlikely to have a good outcome.
Basically, the whole certification process on the MAX has been called into question. Boeing is going to have to convince the regulators that the plane is ok and the regulators are going to have to convince the public that they (the regulators) know what they are talking about.
Since the certification of an airplane model takes a minimum of a couple of years (even for a derivative model), going back and reviewing the MAX certification is not going to be a something that is done overnight, even without any defensive finger pointing.
Another issue is foreign regulators (which were the first to ground the MAX). A number of them see the US FAA as the gold standard and say "If it's good enough for the FAA it is good enough for us." If they come to believe that the FAA is out of their depth and not the standard they think it was, there is going to be a whole lot of pain to get these foreign regulators to approve the MAX again because they won't want to rubber stamp the FAA certificate for their country.
This is a worldwide problem for Boeing. Solving it in the US with the US regulators will help their case a lot, but it will not automatically be a free pass everywhere. The majority of MAX orders are to foreign carriers.