Updating iOS using cellular data

No, you must use Wi-Fi for iOS updates. This is a user-to-user forum, and no one has the authority to answer a "why" question. But it's most likely because if millions of users update their phones using cellular data around the same time it would crash the cellular networks. Cellular network capacity is very limited compared to direct ISP connections. If you've every tried to use a cell phone in a real emergency like a hurricane and been unable to get a connection, that's the reason.

Data networks can also be mildly unreliable, and to avoid corrupting downloads and software installations, running on a more stable network will just increase chances you won't lose internet connection mid-download.

It's the same concept that updating through iTunes is even more stable because of a wired connection to a program that has direct connection to Apple's servers.