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.
(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)
I don't think it's an obvious choice at all.
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.