The pattern that is usually recommended in these cases is the reason ... is that .... I would therefore use Sentence 1 with that one modification, thus: Perhaps the reason why I underestimated this module was that I never thought of it in a particular context, and in this case, education.
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kindaichi000So I was just wondering if it is gramatically wrong.No. Your first sentence is not wrong from the viewpoint of grammar. It's better to say that it doesn't conform to the usual recommendations for good usage, which is more a matter of style than of grammar. ... reason ... is/was ... because ... is not considered good usage.