Because hes an old fashioned guy that doesn't understand anything technology related. That's why he missed every good tech stock out there. I bet you he doesn't own a blackberry.
Considering that he buys stocks to hold for a very long term, you should expect his most recent picks to underperform somewhat. Find me any money manager who has not missed a single great stock. Surely Buffett inhabits that narrow grey area between "profits from every single uptick in everything" and "overrated and washed up".
Its not what he missed, but what he has picked. A lot of his portfolio holdings have underperformed. There are 100's of large cap companies he could have invested in and beat the market. Instead he keeps buying the same old crappy consumer stocks that have done badly since 2000.
Today cnbc runs in a tape loop about buffett playing actor on "All My Children". ...I bet you think I"m joking too...