why is night shift not supported on iphone 5

Apparently, Night Shift is coded in 64-bit. Any iDevice not being a 64-bit device will not be able to run it.

"The Night Shift feature appears to be limited to iOS devices that have a 64-bit processor. That includes the iPhone 5s and later, the iPad Air and later, and the iPad mini 2 and later."

Quote from http://www.macrumors.com/2016/01/11/apple-ios-9-3-night-shift/

Only the 64-bit devices are able to get the Night Shift feature. All the devices that are on the old 32-bit CPU cannot handle the feature, so it's obviously been left out of those to keep the device for messing up. There is some more details here: http://www.redmondpie.com/ios-9.3-night-shift-mode-compatible-iphone-ipad-ipod-t ouch-devices-list/

Because the hardware cannot handle it.