Why were edwards curves given distinct key types, aren't they EC keys?

[Sam Roberts]

It seems like they are EC keys, with specific curve designs, and that
also have some algorithms designed specifically for them, like EdDSA
-- though it looks like that alg is being generalized to other curve
types (https://tools.ietf.org/html/rfc8032#ref-EDDSA2).

What about them makes it necessary to make them distinct, both from
each other, and EVP_PKEY_EC?

Thanks,
Sam