"Why is this answer incorrect?"
'んです' is popular than 'のです' in the conversation. But 'のです' is original correct form. You can confirm about it on the plural websites, I think.
one is :
http://maggiesensei.com/2010/09/08/request-lesson-when-and-how-to-use-%E3%82%93n-%E3%81%AE%E3%81%A7%E3%81%99%E2%86%92%E3%82%93%E3%81%A7%E3%81%99%EF%BC%89/ (But sorry, I don't read all of this.)
If you agree me, report it.
So "x desu" asserts that x is a fact, but "x n(o) desu" is what you say to explain that x is the reason.
That's right, the version with んです or のです indicate a reason for sth and sounds more explanatory than simply "x です".
For an example someone could ask you "Why don't you buy it?" and then you will ask him "お金がないのです" (because I don't have money) or more conversational "お金がないんです" (what means the same, but makes it sound more casual) It is worth meaning that you can also say "お金がないんだ" which is even more casual than previous one.
One important thing - this explanation is only my own understanding of the matter and is based on maggiesensei page(he makes it even more clear than me :3). So thank you!
It's a contraction of のです, where the の turns what came before it into a noun phrase. You can think of の here as meaning "the one" or "the thing". As in "it's a different one".
Is there a reason どうして is required instead of なぜ ? I'm not really familiar with the difference.
I almost feel like Duo is trolling the discussion section with this one. "Let's teach them a sentence they're gonna use ALL the time."
Is んです needed to turn ちがうinto a noun? I'm surprised that it's not accepted without the ん.
This is a question I find myself asking Duo almost every day.