Short answer: To make coverage accessible and fairly priced regardless of health, the ACA barred pricing based on health status or gender; allowing only limited variation for age, location, tobacco, and family size.
Before the ACA, insurers in the individual and small-group markets could underwrite on health history, charging sick people more or denying them outright. Modified adjusted community rating was required to spread risk across the broader pool: insurers may vary premiums only by age (up to 3:1), geography, tobacco use (up to 1.5:1), and family size, never by health, gender, or industry. The goal was guaranteed access to coverage at predictable, non-discriminatory prices.