They don't use updates to enable events (unless an update is actually required to add a new event to the game), they use the servers; the worldstate. The issue with Plague Star is there are crashes currently related to it that need fixing before they put Plague Star live otherwise it would be unplayable. What's the point of launching an event nobody can play?
Additionally, updates/hotfixes can't just be pushed out on consoles like they can on PC. That's not allowed. Code changes must pass certification before they're pushed out to players.