Not everyone is going to get an invite, just how these things go. Being the first to join, one of the first to play CE or having the best spec PC doesn't mean you're going to be invited. Just because you feel you should be invited doesn't necessarily mean you actually should be.
343 haven't, and likely won't, release the criteria they look for during flights because a lot of that can change between builds depending on what they need to test. They will pick people who have been invited previously and have a good track record of providing feedback, which makes sense to do. The rest of the invites will be taken from the Insider pool that matches whatever criteria they need - whether it's lower spec machines to test performance or players with specific GPUs to test bugs.