Free trials of PreCheck were just a standard promotion, except not

Just like the 1-month free trial you get for other memberships, PreCheck free trials were intended to drum up business. However, unlike a private-sector free trial, where the company is always trying to increase its customer base - and where there are competitors that can entice customers away - the TSA has a target number of PreCheck customers, apparently. Why else would they be cutting back on the free trials?