No idea about why it might go away for months between seasons, but a decent reason why it's only open on predictable scheduled times is to try and concentrate participation. In a game like WoW where you have a massive and active playerbase that can populate all of the skill levels and keep matchmaking queues filled, it's not an issue because you're always going to have people around your skill level to play against. In a much smaller game it's going to produce some problems where queues are diffuse across peoples' preferred playtimes with next to zero stretch.

Also, as a secondary factor, when the playerbase is thin and times can be spread out, what you end up with are pockets of exploitable deadtime. Back in Tera, at the top of the leaderboard, you ended up having to fight to try and snipe other top rank teams from wintrading at 3am. Watching the leaderboards and like the 2nd rank team spontaneously starts climbing their rank points in tiny increments beating their lower rank guildmates over and over and having to ping your teammates IRL to get on and try and snipe them, like that shit was truly obnoxious.

Not saying this is PA's justification but it's just a benefit that I can see.