I heard something from twitter during the beta phase that it's complicated to do so since they want the ping of every person to be as low as possible. If someone from the US region would play on the EU serverser, they'd have about 120 to 200ms ping. With this ping you can either have a good advantage when peeking or a disadvantage when being peeked.
I think that cross-region play won't happen in the near future (at least in the upcomming ranked mode, maybe for Spike Rush or Unranked).