Reason for
We usereason for +noun phrase, notreason of:
What’s thereason forthe delay? We’ve been waiting more than an hour.
Not:What is the reason of the delay?
There were severalreasons forhis strange behaviour.
We normally only useofafterreasonin the formal expressionfor reasons of:
The government claims the new law is necessaryfor reasons of national security.
Reason why
We usereason whybefore a clause. We often omitwhy, especially in statements:
Thereason(why) I didn’t contact you was that I was only in town for a few hours.
We don’t sayreason because:
Is there areason whyyou just listed people’s initials and not their full names?
Not:Is there a reason because you…
See also:
Whenas a relative pronoun
Reason that
We usereason thatbefore a clause. We often omitthat, especially in statements.Reason thatis less common and slightly more formal thanreason why:
Thereason(that) we need new guidelines is that the present ones are just not working.
Onereasonthatthe panel could not decide who was the best person for the job was that the interview times were too short.
We do not usereasonin the plural with athat-clause:
There are severalreasons whyI don’t like the book.
Not:There are several reasons that…
Reason+to-infinitive
We can usereasonwith ato-infinitive:
There’s noreason to besuspicious – everything, is perfectly normal.
You have everyreason to demanda guarantee in writing. I would do that at once, if I were you.