I think in all my time of playing WoW I've only been invited to two betas. One was for Wrath and one was for MOP and that makes perfect sense as I was the most active during that time and always had an active sub. Now I play occasionally on and off and haven't gotten a beta invite since.
It's pretty obvious they don't just send them out at random, they prioritize people who have given them the most money haha.
They just use the PR term "in waves" to make it seem like anyone can get it at some point.