why is that? I've heard bon matin before. Is it a formal thing? or is bon matin just wrong?
You may have heard or read "bon matin" but not to wish someone a "good morning!"
"de bon matin" means "very early morning" an it is a bit literary.
Bon matin would be used like, "I had a good morning (J'ai ete un bon matin)" Bonne matinee is like bonne journee...it is wishing someone a good morning/good day.
Please note: "J'ai EU une bonne matinée" or "j'ai passé une bonne matinée" (not a bon matin, anyway). "bon matin" is used in the expression "de bon matin" meaning: early morning.