We observe every update to the limit order book of S&P 500 E-mini futures, the world's most traded equity derivative, over 2009-2025. Exploiting previously unused information in a public data feed, we decompose market depth into the number of liquidity providers and the quantity each supplies. Dollar depth fell 80% over the decade to 2022, and since then has not recovered. The drop is due to the number of providers, not the dollar amount each quotes. To understand this, we model the choice to provide liquidity. Providers enter when profits cover a participation cost; scarcer risk-bearing capacity first draws them in, then drives them out. Rising participation costs can account for the secular decline while weaker demand for liquidity cannot, since trading volume has held up. In exchange-traded markets, participation has become an arms race in speed, talent, and data. As the number of providers has fallen, the price impact for a given amount of order flow more than doubled.