Several reasons you'd have got denied :

- You lack the ability to show that you can accurately detect hacks (proof would be demos submitted and subsequently banned by an admin).

- You're not active on the servers/forums enough.

- You're too active on the forums (majority of posts constitute as spam).

Or some other reason that you wouldn't be told. Just submit demos of people hacking, and as long as they are actually hacking and they get banned, you're likely to be considered for admin.