Why is the S&P 500 considered "The Market"?

[Duck Rabbit]

In the investment world, one's performance is always compared to the market. Why do we consider the S&P 500 to be the market? Wouldn't the Wilshire 5000 or some other larger index be a better representation of the market since the S&P doesn't include small-caps? I realize that certain funds that invest in specific asset classes compare themselves to the appropriate benchmark indeces. Thanks in advance for the clarification.