It is Persistence
Do some people have the magical skill to find vulnerabilities with ease while others don't! Of course not! I disagree with the whole tendency to believe that technical understandings is all that is needed to find vulnerabilities.
It is mostly persistence that plays a role. Most of the researchers I know have almost zero knowledge on the subjects they dive into. The knowledge builds up with the time and by being more persistent on the topic of exploration regardless of its difficulty level.
Simply put, the more time you spend on something, the higher the chances to find oddities. It sound quite logical, don't you think? Countless of examples already exist to prove my point.