I understand, it sucks. I’m not in the beta either and would love to be.
I obviously can’t say for sure as I don’t represent Blizzard, but looking at this logically one of the issues with having more beta testers is that there is more information that has to be sifted through from the increased amount of reports that would be coming in. For efficiency, it’s probably better to have a smaller amount at first, and a larger amount for the stress tests to specifically test the issues caused by high population and high demand separately from the issues of quests, dungeons, NPC placement etc.
Additionally, it might be better to keep the beta more closed to prevent people from getting burnt out from gameplay. Lots of players might be less excited for classic release, if they’ve played the heck out of the beta.