Trigger Event
Researchers theorize that for every person with powers, there’s one to five people with the potential for powers, who haven’t met the conditions necessary for a trigger event. One need to be pushed to the edge. Fight or flight responses pushed to their limits, further than the limits, even. Then the powers start to emerge.
The trigger event is usually a very traumatic experience that cause deep psychological scars. Which may help to explain why the villains outnumber the heroes two to one, or why third world countries have the highest densities of people with powers.
People who have parents with powers don’t need nearly as intense an event to make their powers show up. (e.g. Glory Girl got her powers by getting fouled while playing basketball in gym class)