Because bitcoin XT is not an altcoin project. It was released a long time ago - before the blocksize debate - by Mike Hearn in order to release some extra features he had trouble getting added to Bitcoin Core.

It was and still is used by many people for purposes other than changing the blocksize. It wasn't until later when the blocksize limit debate came around that Mike Hearn got the idea of using this client to change the blocksize limit.

It most definitely is a Bitcoin client right now, it's not fair to censor it. Maybe if we didn't censor it then people would understand the difference between bitcoin core and bitcoin XT better. I honestly think much less people would be running it if they had access to more information. Most of the people I talked to who are running it are doing so because they have been misinformed about things. Note that I'm tlaking about the client (software) here and not the currency itself. I would think that topics mainly about the currency go in altcoin, but a post by someone saying they switched their Bitcoin server from Bitcoin Core to XT should be allowed, at least until XT changes to another currency, if it ever does.