Because your sentence negates a different thing. The sentence above says "not the girl", and you are saying "not there". Those are two very different statements. Hungarian puts "nem" in front of the thing it wants to negate, as opposed to English that usually does it just by emphasis.
Very good, except when you come up with "nem van" together like this, you must replace it with "nincs".
"Nincs ott a magas lány."
But there is one more possibility here. You can either kind of negate the "existence" of the girl - that is what the above sentence does -, or you can say that the girl "exists", the girl IS somewhere, except not there. As in "The tall girl is not there but here."
"A magas lány nem ott van, hanem itt."
You can clearly see the difference in Hungarian, as
"nincs" = "nem van" - the verb "van" is being negated, and
"nem ott" - the place, "ott", is being negated.
The English sentence "The tall girl is not there" blends these two meanings together, you have to play with emphasis to convey the meaning.
This sentence illustrates the biggest problem with Duolingo. Since we would NEVER say the sentence as it is translated, it makes me wonder if the Hungarian sentence is also flawed. Should I use this Hungarian sentence in real life or is it as horrible as the english sentence and I would sound like a fool saying it to a Hungarian? It is impossible to know. Serious structural problem.
That's an unrelated (and slightly nonsensical) sentence. "The girl is not tall here." Why? Is she tall somewhere else?
Remember? Focus comes right before the verb. Now, you can hardly ever come across a situation where you don't want to focus negative information (why would you just casually note something doesn't hold? either say what does hold or why even mention this shallow piece of information). Therefore, focus lasts from nem till the verb.