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.
I know Mike. Poor devil. His ego is bigger than the universe.
So, if it is released by Mike it can't be an altcoin project and should be allowed to post everywhere on bitcointalk because nobody is willing to post on bitcoinxttalk. Any other arguments?