Why does the youngest child ask the 4 questions on Passover?

It doesn't. The four "questions" introduce the "Magid" or recital of the Haggadah.
The Magid stage of the seder is preceded by the Sanctification, then the first washing
of the hands, then the eating of the parsely dipped in salty water, and then the breaking
of the middle cake of matzah. These steps are all conducted before the four "questions".