Pope John Paul II in writing on the subject stated that the Church has no authority to change what Jesus did, as he only ordained men to His priesthood, the Pope had no authority to change it.

It is a similar issue with the other sacraments. Only Grape wine and wheat bread (levened or unleavened, depending on one’s rite) may be used for the Eucharist.

Only potable water for Baptism.

Only a priest or bishop who is physically present for absolution. Not over the phone or internet as some denominations claim.

Why, well because Jesus established the Sacraments, and while the form is subject to the Church, the Matter was established by Christ Himself.