First, informal writing uses “reason why” or simply “reason” — for example, an email to a friend or fictional writing. Second, formal writing will use “reason that” or skip “that” and use only the word “reason” — for example, official emails or academic essays.

One of the most common replacements for “this is because” in professional writing is “due to.” It’s a better way to say “this is because” that shows why something happens. Generally, it’s a more clear and direct choice that shows why one thing leads to another. More often than not, this phrase works best in an essay.