Because it would have helped save a massive amount of money without increasing security risk, in any statistically significant way, for passengers on US-originating flights.

Eliminating the massive amounts of people subjected to strip search machines and to "shoes off/laptops&liquids out" would save lots of time for lots of passengers in a country where time wasted in the name of security costs the national economy money.

PreCheck LLL type screening at airports should be the default passenger screening type by the TSA. Making it free for passengers would thus save a lot of money.