The reason why he was rejected was because he was too young.
The above sentence is improved by replacing that in place of because. My query is why because is not the correct form here?
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'Because' is redundant. It lets the reader know that the next part of the sentence is the reason for the previous part, but the subject of the sentence is already identified as 'the reason', so it's unnecessary.
Including both sounds stilted, and is officially incorrect (though many people find it acceptable). This article (which I have shamelessly appropriated from this ELU answer to the same question) goes in greater detail.