"prime reason" means the primary or main cause or motive behind something. It refers to the most important or significant reason for why something happens or is done. Full definition
They cite salary, working conditions, lack of control over decisions that affect their teaching, and lack of respect as prime reasons for their decision. (socialstudies.org)
But in the later stages of life these only emerge as prime reason of trouble! (over50datingsites.ca)
That means the 5 doesn't offer quite as much interior space or cargo room, both of which are prime reasons for buying a minivan. (automobilemag.com)