On a fundamental level, the S&P 500 index is meant to be a benchmark of the market. Journalists, policymakers, investment managers, politicians, regular investors, everyone I know all use the S&P 500 as the benchmark of the US stock market.
If a significant percentage of the market is excluded from the index because they don't meet index inclusion criteria, then then index stops being a useful benchmark.
If a significant percentage of the market is excluded from the index because they don't meet index inclusion criteria, then then index stops being a useful benchmark.