Doors are the toughest part of a car to design; if you pay attention to cars which get a mid-life 'face-lift', you will see that they never change the doors (even when they change every other body panel). There are an immense number of constraints on a door, not the least of which are the hinge and the gasket. When the Model X was announced, my first comment was that the doors were a bad idea, just because of the complexity of the mechanism, and the 'legacy' of door issues on innumerable cars.

Because doors are part of the exterior design.