http://revach.net/article.php?id=4316

Section: Parshas HaShavua   Category: Quick Vort
Parshas Bo: Kli Yakar & Chasam Sofer - Why Does the Tam Of The Hagadah Only Wake Up The Morning After The Pidyon HaBen?

In the Hagadah of Pesach the Tam asks in his boorish manner, "Mah Zos", what's this.  The Pasuk that the Ba'al Hagadah gets this from is at the end of Parshas Bo (13:14), right in middle of the Parsha that talks about Pidyon HaBen.  Why is the question of the Tam of Mah Zos by Pidyon HaBen and not in a Parshas that discusses Korban Pesach, Matza, and Maror?  Furthermore why does the Torah say, "V'Haya Ki Yishalcha Bincha Machar", he will ask you tomorrow, why not today?

The Kli Yakar answers that the Tam is a very simple kid and not curiously intelligent.  As long they are serving him juicy roasted lamb with tasty filling matza on a bed of sharp vegetables that increase his appetite, he has no questions.  He only wishes they would serve him this every night.  He is not a Rasha and does not scorn the mitzvos.  However when it comes to Pidyon HaBen not only are they not giving him anything, but now he needs to pay the Kohen five expensive coins.  This is where he wakes up and says, "Hey! What is going on here?"  But even so, he does it with respect.  He doesn't ask during the Pidyon HaBen, he waits until the next day.  That is why the pasuk says he will ask you  "Machar".

The Chasam Sofer answers that there is a Machlokes HaPoskim whether a father can be yotzei Pidyon HaBen through a Shaliach like every other Mitzva or not.  The reason to say he cannot, is because Pidyon HaBen was because Hashem saved the Bechorim of Bnei Yisroel when he killed all the Bechorim in Mitzrayim.  That was done by Hashem without a Shaliach, therefore Pidyon HaBen can also not be with a Shaliach.

By Pidyon HaBen the Torah tells us the Tam will ask Mah Zos, why can this not be done through a Shaliach.  To that we explain Yetzias Mitzrayim and in the next pasuk we tell him "Vayaharog Hashem Kol Bechor", because it was Hashem himself whop killed all the Bechorim.