Some required core updates, prerequisite upgrades, Samsung sites may have required special modules and then changing the GCI sectors (Sprint tends not to go ahead until all three of its vendors can do something. Latest example - Massive MIMO). Another huge factor was likely the cost of backhaul. They also had to get Son's approval and raise the capital. 5G represents a leapfrog opportunity for Sprint. Most people don't understand how close to bankruptcy Sprint was before Marcelo took over.
Although Samsung phones have trouble with 256QAM (usually flipping between different values making it hard to determine), it stayed steady showing 64QAM during a data session. On top of that download speeds aren't out of the ordinary from what I normally receive. Maybe I need to go to an area with an even stronger signal and see if things change.