Why don't they make Nuclear Reactors flexible so they can withstand an explosion from within?

The type of "explosion" that happens in a nuclear reactor is a steam explosion, There isn't enough flex in the world to contain a steam explosion of that magnitude.

There is no material, flexible or otherwise, that can withstand an explosion like that.

They can and do have safety valves, but the valves might be power operated and power was lost. They could easily build a pressure vessel like slices of an orange that would split apart at a certain pressure but that would probably release radioactivity. A small amount of radioactivity would be far better than the fukashima disaster. You ccould contain the pressure vessel inside a much larger pressure vessel designed to withstand the expanded pressure (which would be much lower) but that would be designing for failure and would also be very expensive.
Higher pressure means more efficiency. Steam at 100 C occupies 746 times the volume of water at the same pressure, but if you increase the temperature of the steam and hence the pressure you can get more energy to the generators. It is all about money

In most atomic reactors, the heat from the reactor is used to heat water, which then produces team (in a one stage or two stage process) which is then used to drive a turbine to generate power. The heat in the water causes pressures to increase, such that rigid vessels and piping are required, and flexible vessels and piping would simply not work.

And this is speaking of a steam explosion. We generally don't have to worry about an actual atomic explosion because reactors are moderated, and if a runaway reaction occurs, the result is a meltdown rather than an atomic explosion.