why do toi think princess pêche, peach is the main princess?

Because she is pretty and girly and better than daisy

because Nintendo made her before marguerite, daisy ou rosalina and plus she the princess of the champignon kingdom :)

because she has mario who rescues her about each day.

Because Shigeru Miyamoto created Princess pêche, peach to be a part of the main cast.

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