First -- and most importantly! -- there's absolutely zero chance that this particular idiom will make or break your GMAT score. There are somewhere around 25,000 idioms in the English language, so studying them is arguably a futile endeavor, since any of them could appear on the GMAT. And it's rare that an idiom is the deciding factor on actual questions, anyway.
For an extended rant on idioms, check out
this article.
aashishlandmark
REASON IDIOM
RIGHT: I have A REASON TO DO work today.
She has A REASON FOR the lawsuit.
This observation indicates a REASON THAT he is here.
SUSPECT:
This observation indicates a REASON WHY he is here.WRONG: This observation indicates a REASON he is here.
The REASON he is here IS BECAUSE he wants to be.I have two questions
1) Is the suspect sentence wrong or why a suspect because cambridge dictionary says it is good to use if "why is followed by a clause" although most of the time WHY is dropped.
2) why the last sentence is wrong .
I wouldn't waste much time worrying about this, but the word "reason" already implies causality. So there's no reason to add the word "why" or the word "because" to the sentence.
In normal speech, I don't think it's a problem to say "reason why" or "the reason... is because", but in theory, it wouldn't be ideal to use those expressions in formal, written English.
But again:
this issue won't ruin your day on the GMAT.
I hope this helps a bit!