[SoftTalker]

Why don't more sites ask for the username/password and the TOTP code on the same form? Is there a downside to that approach? I've only seen it done that way rarely.

[TrianguloY]

Probably because not all accounts have TOTP enabled, so it would be strange to ask for a TOTP if you don't have one.

Maybe with newer services that requires TOTP since day one is a possibility.