Institutional investors dominate the market.
This means that the trading decisions are made by high paid individuals who professionally manage money. This makes it hard for anyone to beat anyone. Institutional investors then trade amongst themselves in what is called after-hours trading. By the morning opening bell, prices have already been adjusted before the typical investor can react to the latest developement in the stocks they are following. Extended hours trading may allieviate some of this - but your typical investor on the West Coast hardly even notices a 3 hour extention if they have a day job.