It really has nothing to do with safety at all; they're also cancelling Airbus orders and sticking with their existing 737NG aircraft. The 737 MAX orders would be being cancelled even if they were another type.
as far as the general public is concerned, "if its boeing I'm not going" is a viable message. There is almost no downside to them dropping the max, given they had to save money.
(I know that the 737NG are boeing. the point is, given a choice of which craft to drop, the one with tainted PR was the obvious choice)
Unless they can sell on the craft they own, or can offload lease structures they are locked into, There is sunk cost. They'd have to wipe out a huge amount of (possibly paper) money to walk off the 737NG fleet. I believe the Max purchase was fed from growth, as well as replacement, but in shrinkage, It would be untenable to acquire a paper loss, and have to spend real money. Yes, the operating cost is lower, and its a nicer plane. But, it also has certification issues, and will now demand (probably) flight crew training and re-certification when previously had not existed. So its TCO cost has risen.
It's been pointed out they've cancelled Airbus purchases too. This also strongly suggests what I thought about PR value is irrelevant: its not about marketing, its about real money. They may well want the Max. they can't afford it.
