Delta was the first airline to merge in the current round of consolidation and become a mega-carrier. As soon as the dust settled on the merger, they scaled back their frequent flier program, figuring that they were now big enough that they could attract customers without it.

United did the same as soon as their merger was complete, although not to the same extent, perhaps due to the problems they have been facing since integration.

American will likely do the same very soon.

Frequent flier programs were born when the industry was cutthroat competitive. It was just one tool used by airlines to attract customers. Now, the big megacarriers have no need for such a program to attract customers, because customers have little choice.

Fun fact: until recently, Air New Zealand charged a membership fee to join their rewards scheme. That's right, you had to pay actual money to sign up, which is unheard of in the US. They were the only game in town. If you were a frequent international traveler to different countries, you didn't really have any other reasonable options, so they could get away with it. I don't think US carriers will go that far, of course.

As the market adjusts to the reduced level of competition, I expect to see most of the airlines gut their frequent flier programs even further. However, this may be retarded if the ULCCs like Spirit and Allegiant expand significantly.