Initially, there was also a college/university requirement. You had to be in a Boston area university. Then they added Ivy League schools. Eventually, any school with an email address ending in *.edu.
https://www.theguardian.com/technology/2007/jul/25/media.new...
At the time, many were under the impression it would continue as such to prevent the proliferation of middle school/high school students from joining and contaminating the site like MySpace.
Back then, MySpace had some horrible user interface experiences. Users would add javascript plugins and themes to their pages that provided too much individuality. At times, the changes were so drastic you could not tell if a user's page was on myspace without looking at the URL.
To this day, Facebook carries a signature look & feel and will never allow any drastic changes to the UI to accommodate a user's distinct style.