Why computer systems do not simply use one single type of memory. Instead they support a memory?

Hierarchy as you know is a way of organizing things in relative order of priority.
So memory hierarchy is an organization of different types of memories in the order of their increasing access speeds.
(It is also an organization of memories in the order of increasing sizes.)

-This is needed because making large and fast memories is extremely costly and would not make it economically viable.
-Caches are fast access memories.
-The highest level cache is typically very small and works nearly at the speed of the processor.
-But as you go down the hierarchy the speeds are slower and they are larger in size to allow accomdation of large amounts of data.