[jimminy]

This is the correct answer and is actually somewhat visible if you also look at the DJIA and NASDAQ figures.

The S&P 500 and DJIA are limited to a fixed number of corporations. The NASDAQ is allowed to expand it's number of listed companies over time.

If you look at the NASDAQ figures it's able to show considerable and relevant growth, as more companies have listed. (Still not at 2000's peak, but much closer than the S&P or DJIA.)