the main motivation is that we want to use Pkarr.org I.e using Mainline DHT to resolve where user data is (like DNS) and Mainline only supports Ed25519 keys, which Nostr doesn't support.

We can't really make a new DHT to support secp256k1 that Nostr uses, because that DHT would be way smaller and more vulnerable to censorship and Sybil attacks.

there are others strong opinions, but honestly if we could have used the same key, then it would have been considered a Nostr adjacent thing like Blossom servers.