If there is a pattern at all it is that "low risk" (without getting into an entire debate about who is "low risk") people get a free taste and then can't live without it so they sign up.

If you consider that Global Entry (which is only $15 more than Pre-Check and is vastly more useful because it moves you through CBP) costs $100 for 5 years, that works out to $20/year. If you fly 4 times/year (in other words, passing through a checkpoint), that works out to $5 to cut the line. If you are a regular flier, the cost drops all the more.

A regular flier who flies a round-trip per week 48 weeks per year pays roughly $0.42 per use for Pre-Check.