Considering the fact that the next Arbiter armor cap reward will be the semifinal one, Arbiter Legs, before the Arbiter Arms and that the full vanity reward armor is already made, the answer is simple.

It's one of the last things that is worth waiting for. If they finished giving out the Arbiter Reward armor by 56 cap, the few remaining players would lose interest in the game completely.

In about 2-3 months after the winter festival year-update, a cap raise is bound to come. I believe that is when we will get a new map and possibly the next piece of the Arbiter. Keep your hopes up.