We all know that the Hearthstone beta wipe is just around the proverbial corner, but as of this forum post by Zeriyah, we now know a little bit more about why. In addition to the expected reasons such as rebalancing cards and heroes, it turns out that Hearthstone is also being migrated to more robust hardware that can handle a larger player pool. Yes, one of the reasons you may have not yet received a beta invite is because the servers couldn't handle it.
All that should change post-migration, and there should be more closed beta invites on the way. Thus, as Zeriyah said: brace yourselves! The opt-ins are coming.