Some agents don't think it's worth their while joining: they have more than enough clients, and are doing fine without the AAR.
Other agents might not meet the AAR's membership criteria: I've not read them so can't be sure, but it's possible, for example, that if there's a requirement for an agency to have been in business for a minimum length of time, when an established agent leaves their agency and sets up shop on their own they could have to wait until that time has elapsed before they can apply; or they might have to sell a minimum number of books, or something.
Being a member of the AAR isn't necessarily a good thing: I've known some agents I wouldn't like to work with who were members of the appropriate organisations; but it's not a bad thing. And I wouldn't decide against submitting to an otherwise reputable agent just because they weren't in the right club. Sales record, established reputation, loving my work and personality are far more important.