a key reason why


Below is the context:

"Slowing sales forced developers to slash prices, which in turn knocked out a key reason people were buying apartments in the first place: the promise of making tons of money on the property market."

My question is why the relative adverb "why" usually needed to introduce the following adjective clause is absent in the sentence above? Is it grammatically correct? Or that isn't an adjective clause at all? Thanks a million!
  • Cagey

    post mod (English Only / Latin)
    English - US
    There are many people who consider "why" redundant. "Reason" means "the cause". "Why" also refers to the "cause." These people would prefer the construction as it is, or with "that" to introduce the relative clause. (I am one of these people.)

    However, the construction "reason why" has been around for a long time, and is widely accepted. The American Heritage Book of English Usage says this about "the reason why":
    It is true that why could be eliminated from such examples with no loss to the sense, but the construction has been used by reputable English writers since the Renaissance.​
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