Seabury cites four reasons: that the "MMO" label has lost its meaning, it takes too long and is too stressful to develop new games, it's difficult to play around with creative concepts, and the market is trending toward indie titles anyway. He left to start up END Games Entertainment so that he and others of a similar mind could indulge in "a ridiculous variety of game ideas" instead of treading the waters of the status quo.
His letter is largely upbeat as he focuses on the joys of game development and play, and he thinks that it's high past time studios break out of old mindsets to embrace the new: "We have come to a point where the game concept trumps such insignificant bullet points, and global social connectivity is a given. From a creative standpoint, this is fantastic. Ideas are king once again; the industry feels as vibrant to me as the golden nostalgia years I experienced as a kid growing up through the coin-op and early console eras, except we've finally ditched the nerd-in-basement stigma."
You can read Seabury's full letter over at Kotaku.