When national security requests come with built-in gag orders, little-to-no oversight, and are probably easier to issue/get than warrants (no pesky judges or explaining to do) why would one ever use a warrant again?
We have no idea how easy it is for local law enforcement to pick up the phone and request "support" by having a NSL issued.
I'm comfortable assuming that my local beat officer probably can't signal to his superior to pull an NSL on me and start reading every piece of data my devices are streaming over TLS.