But we don't normally use "you people", except where there might be some confusion. It is unusual and could sound rude. Here, in NYC, people avoid using it. I avoided it, quite unconsciously in this lesson. I have the impression that तुम लोग is normal in Hindi. There is a lack of correspondence in the usage.
But this is a course on Hindi, not English. So, you are being taught the manner of speech in Hindi. I am a native English speaker, I get it, but realize those rules don't apply to Hindi. Different language, different culture, and many different ways of saying things, just as in many other languages.
When learning a language, it is important to learn the way things are expressed in that language and not cling to the ways things are expressed in your native tongue.
In some places of the world, it is rude to put your hands in your pockets, but that's not the case in the US. And while in most of the English speaking areas of the world, it is insulting for a person to give their middle finger, that same act is meaningless in other parts of the world.
Try opening yourself to a new way of being, instead of trying to apply that to your current way. It's an expansion of yourself. ;)
Why isn't "When did you go to Delhi" accepted? I understand it is a second person plural sentence, but in English you can stand by itself. Other courses I've been taking on Duolingo on languages that distinguish between 2nd person singular and plural (e.g. Spanish, German, Italian, Hebrew) usually accept just "you" as a translation, and, when translating from English towards these languages, accept both singular and plural answers.
Also, in English "you people" can have bad connotations: https://en.wiktionary.org/wiki/you_people There are alternatives, e.g. the Latin course uses "you all" sometimes.
Because you're omitting a word. In Hindi both तुम and आप can be plural, but in this sentence in particular, लोग is added.
Despite how Americans take offense to "you people," it's not like that in many places around the globe. Are you here to study Hindi or English? In learning Hindi, it's good to learn how those who natively speak Hindi express themselves. It is not uncommon for them to say "you people" and it does not have negative connotation. I've even heard a friend say "आप पंडित लोग" when addressing some priests at our local temple.
When you learn a language, it's good to also learn about the culture, mannerism, and expressions of the people who speak that language. This will help you to better understand where they are coming from and will lead to progressing you towards fluency.