utterly/ mainly

In the sentence: ‘The play was utterly / mainly hilarious-I couldn’t stop laughing.’
the correct answer is given as ‘utterly hilarious’ but why ‘mainly hilarious’ doesn’t fit?
Thank you for your time!

‘Mainly hilarious’ is simply not a standard combination. The term you would use to indicate that something was ‘mainly hilarious’ but not ‘utterly (completely) hilarious’ might be ‘largely hilarious’.