The economy...
The US economy is still pretty soft. Dumping billions into an abstract asset like spectrum isn't that appealing. Plus, why bid the spectrum up against each other? There are only a few players with that many billions available to begin with. They probably set a ceiling on their bidding. I don't know if the FCC carved out smaller regional licenses that companies without billions could bid on. That typically generates interesting competition in smaller areas. Almost anyone can setup an LTE site these days.
These same operators are hoping that 60Ghz and 70/80Ghz, which are coming with 5G, will solve all their problems and be a lot cheaper. It is all near line of sight though, so the operators are going to need a shed load more sites. FCC has recently expanded the 60Ghz allocation for free use.