It was free to play when that wasn’t a popular financial model for games at the time. The memberships were also much cheaper than WoW and accepted different payment methods that would be more available to minors at the time.

Also no need to install or commit to a guild or clan or whatever. You could just log in, screw around for an hour or ten, and log off. Then log in again from somewhere else. It was miraculous for a kid like me.