Perhaps this is because it is a well documented accident. That plane had the very latest and best flight data recorder available. Plus, there are not as many international political considerations that slow things down since it is a domestic flight and the aircraft's manufacturer is based in the US. The NTSB was sure very early on what caused the Egyptair 990 crash but the investigation dragged on in a futile effort to gather enough evidence to convince the Egyptian parties of the truth. Finally, accidents that are smaller in terms of body count and media coverage tend to get solved sooner.
They have an intact airplane, and all crew + pax survived.
They have fairly reliable witnesses on the ground.
So often in a NTSB investigation, they are literally pulling small pieces of metal out of a smoking hole in the ground, and the only witnesses they have are radar tapes, and the black boxes (once they stitch them back together).