For me, only "that" works. "why" sounds wrong.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
AvangiI agree with Wordy.
Can you give us one of the "tons of examples" in which "why" works?
It was for this reason that / why Coca Cola became legendary.
"It was the reason why Coke became legendary" is a completely different type of sentence, by the way.