All that they are doing is what they normally do for betas."The Mists of Pandaria beta was different because so many people needed an invite from the Annual Pass. This time around, beta will be more limited and only used for testing and feedback."
Does not lend itself to inviting many casual players. They want to invite actual testers and people who provide feedback. Obviously some casual players want to actually test, but that beta mentality is far more dominated by the balance minded hardcore players. Its a much safer bet inviting a 2200 arena player than a random random bg hero in the sense of inviting someone who actually will provide useful feedback.
Obviously in some circumstances the bg hero provides more useful feedback, but when you start inviting in he 1000's of people, the numbers balance out to the 2200 arena player a better bet.