Ok, I'll bite
My impression is that the open beta didn't attract enough of a playerbase, leading to the decision to pull the plug on it. After all, open-betas tend to be less about bug-fixing and more about marketing/market research.
I would even go so far and call it a wise decision if my assumption holds true. A lot of CEOs may still be traumatized from the MMO hype and the countless failed attempts to compete with the market leader WoW. The MOBA scene is not to different from that scenario, Riot Games (LoL) has the market under their control and while several other major MOBAs co-exist incl. DOTA2 (Valve) or Smite (by Hi-Rez, apparently seen some growth in the recent months and was announced for consoles too) there is currently no chance that Riot stumbles anytime soon. Essentially the competition is hard and Blizzard's incoming Heroes of the Storm (currently in Beta) certainly won't make it easy either.
Dawngate had a couple of good ideas however it followed a similar design path as HotS or Strife (also in beta by S2 Games), which basically focusses on accessibility, turning them into direct rivals. Meanwhile Riot isn't sitting around either, reacting to the inbound competition with a lot of content additions, reworks, visual upgrades in the recent and coming patches (some of which incl. takes on elements you would find in these new MOBAs).
In general I consider EAs decision to be rather classy, especially since they even refund the beta-players. Though it denies the world a game featuring an Otter that whacks people with an electric eel