Why are you not replying in Spanish?

Answer 1

'Come mai mi rispondi?' is an Italian equivalent of 'How come you are not answering me?' Italian language speakers may choose not to use subject pronouns other than for clarification or emphasis. The second person singular pronoun 'tu' doesn't have to be used in this instance. The subject is clear from the verb form, 'rispondi'.

Answer 2

The same verb is often used for "answer" and "reply" (contestar).

So it would be --" ¿Por qué no contesta?"