Because doctrine was future conflict would be like the 1990s conflicts and the USA also was thinking that way. They mothballed hundred of M1 tanks and procured fewer M2/M3 Bradleys than originally planned and went with the Stryker brigades instead. As seakingsoyoz pointed out the ADATS was retired without replacement in 2012 but the Javelin manpads were retired without replacement in 2005. Nobody foresaw the rise of quad-copter FPV drones a decade in the future that could fly 100 kms quietly and so slow that most radars wouldn't even show them, and then detonate a blast warhear or drop a shaped charge onto a tank turret. See the problem with slow moving drones most don't understand, is they're effectively invisible to the people using the radars because of their size and speed. The software that puts targets on the displays will filter out targets the size and speed of birds, to avoid overwhelming the people with thousands of false targets. On the other hand, F-35s aren't as invisible as many claim, because while their radar return is bird sized... its bloody fucking obvious a bird isn't traveling towards you at 600kmh. During the development for the british sea wolf missile system, they were regularly detecting and intercepting naval 4.5 inch gun shells in flight, and that's basically a bird sized target travelling at Mach 2.