"The reason that" or "The reason why"

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As Anglika has said all are used, but it is best to avoid this; The reason why students did not score high on this test is... because there will always be people who will criticize it's use.

Sorry, bhaisahab................... I just can't resist............... who will criticize its use.

I am one of those who cringe every time I hear/read reason why!

However, in Tenyson's poem "reason" is the verb, and in
The reason why students did not score high on this test, reason is a noun.

I am not a teacher. Just fussy.

[2006]

As Anglika has said all are used, but it is best to avoid this; The reason why students did not score high on this test is... because there will always be people who will criticize it's use.

Its use is criticized because "why" is totally unnecessary there and it sounds bad.
It's like saying 'The way how to do it is...' or 'The place where to go is...'.