Il motivo è che in seguito ai feedback ricevuti dalla comunità, vogliono apportare alcune modifiche, che comunque non tarderanno ad arrivare.
Jeff Kaplan ha scritto
We worked extremely hard to get Competitive Play in the game for Closed Beta. We've been reading feedback from the community and taking it all to heart. As a result, we've temporarily disabled Competitive Play for Open Beta so that we can rework the system and make it better.
Our plan is to bring back a newly re-designed version of Competitive Play sometime after Overwatch launches.
Thanks for all of the feedback, and thank you for being patient with us while we work to improve the game!