"Explain why could 'only' not be used" is not grammatically correct and it's not idiomatic.
Here's what the Macmillan Dictionary says about why:
Why can be used in the following ways:
as a question adverb (introducing a direct or indirect question): Why didn’t you call me?I know why he’s worried. (followed by an infinitive without “to”): We can start now, so why wait?
as a relative adverb (referring back to a noun and introducing a relative clause): She’s tired and that’s the reason why she’s so bad-tempered. (introducing a clause that is the subject, object, or complement of another clause): Everyone in Dublin is so friendly, and that is why I love the city.
https://www.macmillandictionary.com/us/dictionary/american/why_1
When why is used as a relative adverb - as it is in your sentence - it must be followed by a noun or a pronoun:
Tell me why he did it
Explain why dogs chase cats
This is true even when the sentence has an auxiliary verb:
Tell me why he had done it
Explain why dogs were chasing cats
This is different from sentences that use why as a question adverb:
Why had he done it?
Why were dogs chasing cats?
In your sentence, you seem to be reversing the standard word order, as if this sentence were a question, but it's not. You're using why as a relative adverb, so the standard word order must be used:
That post does not explain why "only" could not be used as an adjective in my second example.