It's been that way since Beta 1.
When you update to 10.3, your filesystem is upgraded to Apple File System. Apple had to write code for that conversion to happen, but they didn't write code to convert your File System back to HFS+. (Why would they?) Since iOS 10.2.1 doesn't know how to use the APFS, downgrading results in an error.
You have to do a DFU (bare metal) restore to go back to 10.2.1.
So for the obvious reason , apple won't let you to restore back to iOS 10.2.1 even if it is still being signed??That results in a "There's a problem with your iPhone.
Reasonable move.