Because you need both WiFi and cellular service to update. You screenshot shows you have no WiFi.
Make sure you:
• Are using a WiFi connection (not a hotspot).
• Have an active cell service
• Have your battery charged at least 50%. Better yet, keep it plugged in during the update.
• Are not using a VPN connection.
• Do not have power save mode turned on.
• Do not have a beta version installed.
• You have at least 6 gigs of storage available.
You can also check out this article for help -
If your iPhone, iPad or iPod touch won’t update
https://support.apple.com/en-gb/HT201435
