I think the biggest selling point for AR-15 is that the left doesn't want us to have them JMHO!!!
They're so customizable.
Why do kids build with Legos?
Why do people tune and tinker with cars?
Why do women love decorating homes?
Why do people build model train set worlds?
People like making things their own and unique. The AR 15 is very ergonomic, effective, fun and easily personalized.
Mini 14 and AR-15 are for all comparisons "roughly equivalent". The AR has vastly surpassed the mini in popularity.