It may be the success of the program... I've seen flights with just a couple full BC fares (though lately with SW bookings even more) Then a few purchased upgrades and 20 or more Elites hoping for upgrades.

Last flight I booked via Southwest on FL with a 2 day prior fare the difference between BS and AT was less than $12. The extra points were worth it alone.

DL's hoping to Upgrade list often is far longer even on smaller cities. On RJs on business routes it can get very close to the entire number of passengers booked. Diamond and Platinum might score. Golds very rarely get UG's even on mainlines.