Good question. IIRC, I did that because I was assuming I’d probably handle not just naddr format URLs in the future. I’ll have another look, and update my nip 89 endpoints if I don’t change anything.
NIP-89 unluckily only supports placing a single bech32 id into url, it can't handle npub and naddr at the same time, not to mention the kind.
Maybe the NIP itself could be improved, but at this point it doesn't fit.