The entire point of bitcoin is that the USERS can choose. Not any one group of devs.
No one is stopping any one else from choosing a different version of Bitcoin to use, especially not the forum.
Keep in mind that this is a private forum, and whatever theymos chooses to be listed here is what will be listed here. He has no obligation to anyone to do anything else.
Classic, unlimited and XT are all bitcoin, and I'm fairly sure they would all be more popular than Core if they would have the same opportunity to be listed as 'official' bitcoin clients.
Core is in no way more 'official' than Classic, Unlimited or XT.
If I made a coin named 'Bitcoin 2', and used almost all of the exact same properties of Bitcoin (21,000,000 coins, 12.5BTC/block, same chain) apart from changed some technical aspects (block size, block speed etc) would that be an 'official' version of Bitcoin? If not, why?