What is the motivation behind the choice of unglazed Porcelain as a Streak plate?
It's white, so the streak colour is obvious.
It's hard. Porcelain is made from aluminosilicates like sillimanite and mullite. Mohs hardness is around 7. Not many geological materials are harder than it, and streak is not a diagnostic feature for those that are harder.
It's rough (i.e. unglazed) so it's really good in breaking tiny fragments from the tested material, thus producing the streak.
It's cheap and widely available.