The reason is PAX Prime. Truth be told, I enjoy DC over PAX. For me, it's the better con due to the people I see there. However, I have to consider what's best for my career in game development.
Earlier this year, Penny Arcade ran a Kickstarter campaign to raise funds which would allow them to remove advertisements from the site. It wasn't a cause I was particularly enthused about, but one of the reward levels covered admission to every PAX event for the next five years. Considering the rising cost of badges, potential for new events, and that it includes the PAX Dev event, it was a deal I couldn't pass up.
The conflict is that PAX Prime and DC often reside on the same weekend. And by often, I mean only once did they ever not conflict.
The relationship to my career is this- DC is not a con that places much emphasis on the game industry. This year was the first year in many that had any notion of a video game track. There was only one dedicated board game vendor present. The number of players in the free play area couldn't have passed 1000 uniques in four days. Right now, PAX is where I need to be in order to grow.