Faction Change on Frostmourne.

Hello. I'd like to ask and possibly get a logical explanation if possible why there is NOT faction change on Frostmourne? What's the reason behind this, if you can buy characters from trade and also u can buy full bis gear from store, why there isn't the possibility of a faction change? For example, my old guildmates are now hordes but I didn't know they are planning on playing. I'd love to hop on horde as soon as possible since I'd like to prepare for the Ulduar competition. Instead, I'm stuck in alliance.
Thanks a lot in advance for the response.