The fare buckets that qualify are typically paid F fares, full fare Y, and one or two other fares that rank just below full Y in terms of wallet busting. So if most of your travel was booked in advance on non-refundable fares, that would explain why so few qualified.

As I alluded to above, my opinion is that unless you are wealthy or have an employer that will pay full Y fares, NW's challenges are typically not too much use--the fare classes required mean you would shell out far more $$$ to do the challenge than to fly 25,000 miles on cheaper fares.