I don't know all the details, so if anyone has a better answer, please post it.
There are several volunteers that spend a lot of time reviewing and comparing wallets. They have checklists of important points they look at and try to be as thorough and impartial as they possibly can.
As you say, there are now hundreds of wallets, each of which update at least every few months. So as you can imagine it's a lot of work no one is getting paid for. It's also pretty ungrateful work as accusations of unfair treatment of a specific wallet is easily made, while at the same time if they list a wallet that turns out to be even more insecure than they reviewed it, or turns out to be malware and people lose money, then people will also be quick to blame them.
Blockchain.info for example has been notorious for bugs in their wallets that have cost loss of bitcoins in some occasions. They were delisted at some point, leading to a lot of controversy and accusations on the forums. I don't know if they are currently back.
Personally I'm happy they err on the safe side, even though that can be a big nuisance for wallet developers.
I would assume the short answer to your question is that the work simply hasn't been done yet: not all (latest versions) of all wallets have been tested.
As always in open source: you're free to volunteer to help do the work.
1Jamal, just so you know, this question might be contentious. Due to the on-going blocksize debate, accusations have been made about the objectivity of the maintainers of bitcoin.org. Without taking sides, I'd like to let you know to take any answers with a grain of salt. – Jestin – 2016-05-18T21:49:37.557
Thank you Jestin. I am taking no position on the blocksize debate or raising allegations of bias here. Regardless of whether bias exists (or not) it would be helpful for me to know the criteria that is used for listing new wallets. – Jamal Wilson – 2016-05-18T21:54:16.223
@Jestin That's a pretty low accusation, especially since Wallet Review doesn't have anything to do with the blocksize debate. As well as the fact that history has proven them completely right in hindsight, now that the block size debate is over. On the other hand, it makes much more sense to warn against any information on bitcoin.com, which promotes attacks on the Bitcoin network, directs newcomers to the absolute worst wallet by definition and accepts money for having a wallet listed at all. – Jannes – 2016-05-19T11:46:41.413
@Jannes, I didn't mean anything by it, I was just concerned over the potential flame-war spillover from other forums. I wanted to head off any answers like "because the bitcoin.org guys are biased" and the like. Newbies don't really need to hear those fights. I don't actually have an opinion one way or the other on that particular site, but I've certainly heard the accusations. – Jestin – 2016-05-19T13:06:39.863
@Jestin I didn't mean to say you made that accusation, just that the accusation itself, whoever makes it, is low. You're right to cut short any such flamewars. Thanks. – Jannes – 2016-05-19T16:22:10.900