I would guess that on any given flight most passengers are vacation/leisure flyers and the cost for Pre Check just isn't worth the cost verus the number of flights they take in a years time. Yes, lots of people fly everyday but for the most part they are different people each day and that is where TSA Pre Check misses the mark.

If TSA wants vetted passengers then do it for free, require no in person interview, and use current law enforcement databases to make determinations. Doing so would allow for Pre Check style screening for the majority of travelers and let TSA focus their energy on the remaning unknown or higher risk passengers.