Because Game Freak said so, unfortunately. No one knows why they forbid tradings of event-only Pokemon. --- "You got to be careful if you don't know where you're going, because you might not get there." - Yogi Berra
All event only pokemon can't be traded on GTS. you can wondertrade or 1 on 1 trade it though. --- 3DS Friend Code: 3007-9110-9207 Pokemon X Ice Safari Feel free to add my FC. Just PM me so I can add yours.
but it would help to get these legends off and from what I see on GTS mew , genesect , darkrai are in high demand specially for yveltal ( which I don't have) --- have yet to figure out how to double quote