So many people make this mistake. My very favorite Dodgers announcer, Vin Scully, whose English was really excellent, made this error. And he is not alone. I heard it three times yesterday and had the news on for only an hour.
Once you say “the reason,” all other words implying the reason are unnecessary.
…The reason why is that she was ill. ?
…The reason is because she is ill. ? [This one leads the list!] …The reason is on account of she was ill. ?
…The reason is due to the fact she is ill. ?
The ONLY one that is correct!
…The reason is that she was ill. ?
Or you can omit the word “that.”
Just a little PSA from one who cares about the language.