There are several reasons why most stars are found to be in their main sequence:

1) The mass needed to become a supergiant or larger is very rare. Most stars are far smaller in mass.

2) The main sequence is the longest phase in the lifespan of a star. A main sequence can last several billion years.

3) The universe itself is between 13-14 billion years old. That is not enough time for a large number of stars to become dwarfs.