why do 你 think princess 桃子 is the main princess?

Because she is pretty and girly and better than daisy

because 任天堂 made her before 雏菊, 黛西 或者 rosalina and plus she the princess of the 蘑菇 kingdom :)

because she has mario who rescues her about each day.

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

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