Remember that rules and legislation come from people called politicians. These people are lawyers, not scientists - just 21st century Snake Oil salesman. They'll listen to the press, their friends etc. but rarely to scientific argument. As consequence, they believe that more engines equals more safety and therefore less stringent operating conditions can be allowed. Therefore, aircraft are still built with four engines when two would have done. Unfortunately, the most likely reason for diversion is icky pax and other non-technical reasons which are nothing to do with the plane. And when you look at four engined ops, what is the most worrying? Pressurisation failure at the wrong place!