It often costs far less than 6 mana despite being an incredibly powerful threat, and its a devil to interact with between the immense ward cost that makes it hard to target and the triggered ability that makes it impossible to block
Fortunately, this is exactly what Doctor Doom, King of Latveria does, making Green Goblin a premium upgrade
Part of that is that people are building a lot and the game is popular, and part of is that EDHREC spent the last year trying to get the daily scraping routine to finish faster, resulting in data days and sometimes weeks earlier than before
Now, some Modern decks can win the game before most decks can even cast Jace, so it was only fair that this planeswalker was unbanned
The plan is for planeswalkers to be mythic rare by default, but if a design team feels that a rare (or uncommon) planeswalker would serve the set, they have access to it