"Why do they not visit you?"
Translation:¿Por qué ellos no te visitan?
The "no" is always coupled with the verb and the subject does not come between them. The subject precedes no (and nunca) in this kind of sentence, and you will often see the subject follow the verb when the sentence is preceded by an interrogative word/expression such as Por qué, cuándo, dónde, qué (etcétera).
The first sentence you gave was incorrect because ustedes came after no and should have come before it. I know people often say that placement doesn't count as much is Spanish as it does in English, but I find that placement often does matter. Good for you for experimenting, though. Best way to learn.
Te and tú have different meanings, but both translate to "you" in English. Those "you" words actually have different meanings too, even if they look and sound the same. In phrases like "You visited" and "I visited you", that pronoun plays different roles.
This is explained by grammar. Most of us don't like grammar because it is hard and uses technical terms. But there is a reason for grammar. It speeds learning once we know it.
Basically, Spanish and English are very different they way they use pronouns. That can be confusing because we don't expect it. But it is so. Spanish is different, and we can learn those differences.
Check out this link.
You can find many more useful sites like it by searching on "Spanish pronouns" or "Spanish object pronouns".