It might help to think of the Duoligo sentence as a fragment of a larger group of sentences: I said to her "This is an interesting box. Why don't you open it?"
However, the structure of the Duolingo translation seems fine to me "as is" when used for emphasis in spoken language, but that may be regional.
If I see a loose dog, I might say to someone, "This dog, is it yours?" Of course, "Is this dog yours?" would be more graceful, but I might not be thinking of that if I'm afraid that the dog might be hit by a car. So, my point is that there are different ways to express things depending on the region and situation.
I've seen quite a few of these types of sentences in the new material and my conclusion is yes. Duo has deliberately included both "esta caja" and "la" in the Spanish sentence, so they do want you to use both "this box" and "it" in the English translation.
Your answer was marked incorrect for leaving out the word "it" in this sentence. "Por qué no abres esta caja?" = Why don't you open this box?" or "Why aren't you opening this box?"
In these sentence, it's not so much that they are teaching the meaning of individual words but they are trying to help people learn different types of sentence structures in Spanish. Having both a named object "this box" AND an object pronoun "it" seems very redundant in English so I do understand the tendency to streamline the sentence. However, that is the point. Spanish often does have redundant object pronouns in the same sentence with a named object.
Duo has done some lessons where simply asking a question implies the presence of "poder" without actually needing to use it, ex) "Abres esta caja?" would be "Can you open this box?" not "Do you open this box?" In this case here, why can't it be "This box, why can't you open it?"