And the reason is actually pretty simple as is made clear by Business Insider -
"... Right now, if you cancel your Max order, what are you going to substitute for? Airbus would love to have more business and to take customers from Boeing, but the A320 family and the 737 line are sold out for years into the future. You could probably find the occasional airplane, but switching a whole large fleet order would be quite difficult.
Moving from Boeing to Airbus aircraft could be particularly challenging, since Airbus has less production capacity than Boeing.
Beyond capacity constraints, airlines make a series of investments and decisions when they order an aircraft — like training employees, buying the equipment and parts necessary for maintenance, and designing a schedule based on the plane's capabilities — that can be difficult to untangle.
As an airline, when you commit to an airplane, it's like getting married in a place where divorce is very difficult..."
https://www.businessinsider.com/boeing-737-max-8-orders-unlikely-canceled-experts-2019-3