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Early design choice. The ability for people to change their username was a new thing, and people were changing them a lot during the first several months, and that influenced the search as I wanted to make sure that searches were cut 'n' pasteable / bookmarkable... and wouldn't break when someone changed their name.
People change their name far less now, so probably could be revisited.
Why does the search need authorId? It seems counterintuitive to require that the person finds the number from the name, then searches on that. A little like making you search for the IP address behind a domain name, if that makes sense.