This is what my grammar sources tell me:
(Without getting into all the technicalities)

DUE TO is basically used is as an adjective. It means "the result of" or "resulting from." It is only and always used following a form of the verb "to be."
His defeat was due to his overconfidence. ("due to" modifies the noun "defeat")

BECAUSE OF is basically used as an adverb

Doctor DBECAUSE OF is basically used as an adverb. It means "as a result of." It is never used with the verb "to be"

Your grammar sources are out of touch. There are over 700 COCA citations for '... was because of...'.

Well, a lot depends on the rest of the sentence. I wouldn't say, for example, "His defeat was because of his overconfidence," but I could say "It was because of his overconfidence that he was defeated."

That doesn't mean that the usage is correct. And, as you see in the following post, "was because of" can be a misleading citation, because any sentence can be re-arranged to place a "was" before "because of."

Take the basic sentence: "He was defeated because of his overconfidence." If you re-organize this, you get ozzourti's sentence: "It was because of his overconfidence that he

Doctor Dit would be more correct to say "It was due to his overconfidence that he was defeated."

Who says it's 'more correct'?

Doctor DThat doesn't mean that the usage is correct. And, as you see in the following post, "was because of" can be a misleading citation, because any sentence can be re-arranged to place a "was" before "because of."Take the basic sentence: "He was defeated because of his overconfidence." If you re-organize this, you get ozzourti's sentence: "It was because of his overcon

Thank you for your help
Best regards
mazh