Increasing reputation levels is no longer tied to beta status. The primary indicator of readiness is having x users with n reputation. Many sites that aren't beta don't have the users to support higher reputation levels and still get things done. So, each site should decide for themselves whether they think raising the privilege levels is appropriate for their site.
In my mind, this is a valuable separation. The number of users needed to perform certain tasks and the reputation needed is somewhat arbitrary but it's particularly important that you have those people on site who can participate, else the site will fail to function properly.
The two most obvious examples are with close votes (500 vs 3k) and delete votes (4k vs 10k). If you don't have 5+ active reviewers on the site with 3k reputation, you will never close a question without moderator assistance. If you don't have 3+ active users with 10k+ rep who can vote to delete, you're unlikely to succeed in deleting non-answers and low quality answers without moderator assistance.
Y'all have taken a huge blow, through no fault of your own. Looking at numbers, I'd say you're somewhat on the cusp. You have enough people but I'm not sure that you'll be able to keep up with reviewing at the higher rep levels. You have a total of 36 users with 3k+ reputation but 42% of them (15) haven't been on the site in the last 30 days and 70% of them (25) haven't been around in the last 7 days.
Over the last 30 days, only two 3k+ users have reviewed more than 10 items in the close queue - the top five reviewers are all under 3k. If I raised your privilege levels, what closing was getting done would essentially stop. NB: Reviewers, specifically. This doesn't look at people voting to close on the post directly.
In general, you're doing OK with your reviews. Over the last 120 days, 241 posts were flagged or voted closed and y'all have handled 71% of them. Many sites do far worse. But this would doubtless drop dramatically with higher levels.
Until a point in time where the majority of the review activities are being handled by 3k+ users, y'all will stay at the reduced reputation levels. Please don't see this as a badge of shame or indication that you weren't ready to leave Beta. Our goal in removing the Beta label was to indicate to everyone that we believe small sites are great and stable. This means that sites may keep the lower rep levels indefinitely.
And, if you ever feel like you're ready and we don't notice, let us know. Have your mods ping a CM or use the Contact form to get us to look into it.