When the iPhone 4, 4s and 5 came out Sprint only had certain bands. Sprint didn't own Clearwire yet and Sprint wasn't in turn owned by Softbank yet.
The iPhone 5c and 5s were pretty much finalized and in production by the time Softbank completed it's buyout of Sprint (July I think). Sprint on it's own only operates in the 1900 frequency.
I don't pretend to know much about bands, but 800 and 2700 are coming now that Sprint has Clearwire and has shut down iDen (the iDen towers are being repurposed). The first of Sprint's triband devices are already being prepared to sell. But it was too late for the new iPhones. Expect though the iPhone 6 to have Sprint's new bands.
Note that the buyout of Sprint is a cosmic joke on Sprint. Originally they had the 2700 band because they got it from Nextel when they bought them out. Guess who Sprint sold the 2700 frequencies to? Clearwire. All the bazillions of dollars Sprint had to pay to get it back
sheesh!