Why the programs and data cannot be stored in the main memory permanently?

This arrangement is not possible for following two reasons:

1. Main Memory is usually too small to store all needed programs and data  permanently.

2. Main memory is volatile storage device that loses its contents when power is turned off or otherwise lost.

Thus, most computer systems provide secondary storage as an extension of main memory. The main requirement of secondary storage is that it be able to hold large quantities of data permanently.