The Xbox Insider program is much more closed than its big brother, the Windows 10 Insider program, with participants having to be invited by people already in the program and there be a limited number of places, as decided by Microsoft.
In addition to this, being part of the program no longer means that you'll necessarily receive builds, or as often - as the Xbox team added 'rings' to the program, which decides how many builds you'll receive, this again however, is a selective process and you'd have to be active in the program to get up the rings.
Now, Mike Ybarra of the Xbox team at Microsoft, took to Twitter to announce that they're adding more Xbox Insiders over the coming days:
We’re going to expand the pool of Xbox Insiders in the next few days to get more coverage. As always thanks for helping to create Xbox.
— Mike Ybarra (@Qwik) February 4, 2017
So, keep an eye on your status for the Xbox Insider program and you may just be lucky enough to get in. It's unknown how far the program will be expanded, in terms of how many new people will be added.