Not really surprising.
The two airlines & countries whose aircraft crash are first to withdraw them.
Everyone else looks at the data, 2 new aircraft, both under 6 months old, similar circumstances, other pilots saying they've had similar unexplained nose down attitudes after engaging autopilot (and that's just the stuff us outsiders know aboutI) and quickly follow suit denying access and overfly routes to the type one by one. Airlines who stand to lose money because no airline has spare planes just sitting on the ground also ground them.
Boeing says there's nothing wrong with it, so do Southwest and American Airlines, So does the FAA. It's an American plane, American dollars, and those airlines have a lot of them. Eventually even they realise it's politically and commercially dangerous to be an outlier, catastrophically so should another one go down, and reluctantly eventually follow suit.
Of course it may be nothing to do with the plane, but call me a cynic, it's not going to be quite the same order of play were it an Airbus.