The Reason Is ThatWhy you should use "the reason is that" instead of "the reason is because"
The expression “the reason…is/was because” is quite commonly used by native English speakers, but it should be avoided in favor of “the reason…is/was that.” The word because essentially means “for the reason that.” Therefore, “the reason…is because” is redundant:
The reason I went was because I wanted to see the opening band
= the reason I went was for the reason that I wanted to see the opening band (redundant)
the reason I went was that I wanted to see the opening band (simple and logical)
Common Errors in English Usage: Errors in diction and idiom commonly made by native speakers of English
List of Common Errors in English Usage (PDF): Printable version of the complete list
Common Grammar Errors: A list of common errors in grammar (topics like subject-verb agreement and parallelism) as distinct from usage