Update the iOS software on your iPhone, iPad, or iPod touch - https://support.apple.com/HT204204 - Updating will only offer the highest available iOS version supported by that device. If it says no update is available and you need a higher version you will have to buy newer equipment.
As for why, presumably the hardware is too old to support features offered in iOS11.
Vintage and obsolete products - https://support.apple.com/HT201624