why do you think princess peach is the main princess?

Because she is pretty and girly and better than daisy

because nintendo made her before daisy or rosalina and plus she the princess of the mushroom kingdom :)

because she has mario who rescues her about each day.

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

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