The word "is" is a linking verb, which means that whatever follows "is" must either be a noun or an adjective (because what follows "is" either renames the subject or modifies the subject).
Now then, the word "because" is an adverb, and if "because" follows "is," then what you have is neither the required noun nor an adjective, but an adverb or adverbial clause.
The 'is when' construction won't do. You must use either a noun or an adjective after "is," so you must write "is that," not "is because...."
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