The growth of volume was the result of retail investors. Throughout the bull market of the 1990s and the tech frenzy that accompanied it, retail investors flocked to manage their own money in U.S. markets.
Data provided by Yahoo Finance
This frenzy peaked in 2008, when the financial crisis hit and investors' portfolios dropped by 50% in less than two years. Since the crisis, trading volume has declined significantly.
Lightspeed Trading, an online broker, gives some reasons for the low trading volume.
One reason has to do with market integrity. The perceived decline in integrity of the U.S. market structure has prompted many investors to avoid the stock market.
The May 6, 2010 Flash Crash had a devastating impact on retail investor sentiment, and many institutional traders have said they fear the seemingly increasing instability of the market structure.