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.