You use mainly when mentioning the main reason or thing involved in something. 主要地
adverb [程度副词]主要地,首要地;大部分,大多
The stock market scandal is refusing to go away, mainly because there's still no consensus over how it should be dealt with. 此次股市丑闻历久不散,主要是因为在此事如何处理上尚无共识。