why do Du think princess pfirsich is the main princess?

Because she is pretty and girly and better than daisy

because Nintendo made her before gänseblümchen, daisy oder rosalina and plus she the princess of the pilz kingdom :)

because she has mario who rescues her about each day.

Because Shigeru Miyamoto created Princess pfirsich to be a part of the main cast.

gänseblümchen, daisy doesn't get the same luxury because Super Mario Land was made without Miyamoto's knowledge. (Or so I was told.)