Is it 1) Why aren't you interested ? or Why you are not interested ?
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
So there is no difference . right ?
A better title would have been Why aren't you interested?
'Thread titles should include all or part of the word/phrase being discussed.' (Posting Guidelines)
NOT A TEACHER
I thought that you would be interested in what one expert *** says:
We use the full form in formal [my emphasis] questions or when we require special emphasis to express anger, surprise, etc. [my emphasis].
The expert gives this example:
Have I not asked you again and again to be here on time?
He also says that the full form is used in rhetorical questions (when we do NOT expect an answer). The expert's example:
Are there not more than enough weapons of destruction on earth?
If you accept the expert's opinion, then maybe "Why are you not interested?" shows anger or surprise -- depending on the particular situation.
HAVE A NICE DAY!
*** L.G. Alexander. Longman English Grammar (1988), page 255.