Why can't I say 'Ni vizitu al Ĉino"?
Because that's the syntax of "viziti". "viziti ion/iun" means "to visit something/somebody"
It's a good habit always learn verbs with their syntax.
So the imperative is used with a pronoun for hortatory and jussive clauses?
Vizitu ĉinion. Why not?
That would be OK if you were telling somebody else to visit China, but not for "Let's (short for "let us") visit China". You need something in Esperanto to indicate the "us", that is why it is "Ni vizitu Ĉinion".