It would be costly for an airline to have to retrain almost all their pilots, mechanics, etc on a new aircraft...and to purchase all the associated supporting equipment.
And, the Max has significantly better fuel efficiency, seating capacity, etc than other 737 models. Falling back to an earlier model isn't easy either.
They are banking on the general public forgetting about it over time. Which has happened in the past.
[1] https://en.wikipedia.org/wiki/List_of_Boeing_737_MAX_orders_...