Longman Dictionary of Contemporary English - mainly

~ adv as the largest or most important reason, thing, part of something etc  (Her illness was caused mainly by worry and stress. | The workforce is mainly made up of women. | the mainly Zulu Inkatha Freedom Party) sentence adverb  ("What did you do all evening?" "Well, we just talked mainly." | mainly because)  (I don't go out much, mainly because I have to look after the kids. | mainly due to (=caused to a great degree by))  (Increased sales during the summer were mainly due to tourism. | mainly why/what/how spoken)  (Boredom was mainly why I decided to quit.)