Because the stock market doesn't care how awesome your innovation is. All they see is $$$ in profit. That's why it's an oil company sitting on top now (take your pick of tech companies; I'll bet most/all of them are innovating more than Exxon)
Tech companies are a lot more volatile. Any of their innovations can flop, but the world runs on oil. That's the reason Exxon is #1. They are a rock solid company. Kinda like Philip Morris, nothing like an addicted user base to feed you consistent profits.