If you could remember vividly that here is bitcointalk and specifically has to do with bitcoin discussions before any other things may follows, so including altcoins is simply an initiation for something else the forum may not take as a welcome idea, knowing that all about the forum is expected to be the coin that we are here to discussed, because this is the forum you're dealing with directly on this regard and not other services which may be third party.
It's Bitcoin forum and bitcoin is a currency in the forum for donation and Coppermembership purchase. There is only one time when theymos accepted an altcoin for Coppermembership purchase method, it's Grincoin. After a while like some years, theymos revoke the acceptance of Grincoin because the code for its payment method was broken and he did not have time to fix it as well as there was very limited use of Grincoin by forum users when buying Coppermembership.
It's not the main reason which is what purpose the forum is "Created for Bitcoin and Bitcoin related discussions".