Conventions apparently are really expensive. First you have to pay someone to do the planning, then pay for a location, and then get and pay staff to manage it. The cost and time it takes to make a good convention can add up pretty quickly and even the money printing press that is Blizzard has a budget. Taking this into account, Blizzard has done something that they are very good at with their games; they've delayed the next BlizzCon. They've delayed it in order to devote more time to and resources to Diablo III, Starcraft II: Heart of the Swarm, and World of Warcraft: Mists of Pandaria. Blizzard has decided to hold the next BlizzCon in 2013.
Personally I cannot fault them for this. They have a lot of games on their plate and I have never really given thought to going to BlizzCon anyway. What this convention delay means for me is that I will get Diablo III sooner as they can use the resources from the convention to finish it and their other upcoming games.
I can already hear some people that are disappointed about the lack of Blizzard related tournaments due to this delay. Well there is good news for you. Blizzard is going to host a 2012 Battle.net World Championship in Asia later this year. Ok, so it's not too good for American gamers since it is not held within the country and it will be much more expensive to go compete in Asia then L.A., but at least it is something.