Why did Prophet(SAW) asked people to slaughter 1 goat for girl and 2 goats for babyboys for Aqeeqah? what is the reason to this differenciation?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu`alayhi wa sallam, is His slave and messenger.

It is recorded in the Hadeeth narrated by ʿAa'ishah that “the Messenger of Allah commanded them to slaughter two sheep equal (in age) for a boy and one for a girl at their birth.” [At-Tirmidhi who classified it as authentic]

Scholars also highlighted the legal wisdom behind this Prophetic guidance of assigning two sheep to be slaughtered as ʿAqeeqah for the boy and of assigning one sheep for the girl.

Ibn Al-Qayyim said, “This is a Sharee'ah maxim since Allah, the Almighty, differentiated between males and females and made females deserve half the share of males in inheritance, blood money, testimonies, emancipation, etc. Similarly, the rulings of Al-ʿAqeeqh go the same way.”

However, this does not mean to belittle the blessing of giving birth to females, since Allah, the Exalted, mentioned His Favor upon His slaves as He granted them female babies before doing that with male babies – a fact recorded in the Saying of Allah, the Exalted, {To Allah belongs the dominion of the heavens and the earth; He creates what he wills. He gives to whom He wills female [children], and He gives to whom He wills males.} [Quran, 42:49]

Allah knows best.