Because that's why - if you play Warlords of Draenor while you're at PAX East, you get a beta key for use when Warlords of Draenor enters later testing. So if you're there or planning on attending, you should get on that.
It's interesting they have this for conventions like PAX East but they didn't for BlizzCon - I'm not sure why that is. But have it now they do. Community Manager Lore has tweeted that there will be other opportunities to gain beta access, so perhaps keep an eye out for other convention appearances by Blizzard.