ADHD is a different type of brain wiring... it becomes pathological when it is in an environment that stresses its abilities rather than responding to them. There is a genetic variant, D4D4 7R, which researchers are associating with ADHD symptoms, particularly novelty-seeking, greater cravings for food and substance use, etc.
When looking at two communities in Kenya in 2008, it was found that in the still nomadic community, those with ADHD symptoms were better nourished and functioned more acceptably than their non-ADHD peers; in the village community, those with ADHD struggled more in the classroom and other more structured areas of communal living.
Humans were hunter-gatherers until the advent of agriculture. Hunter-gatherers needed to know a little bit of everything, they needed to be flexible, adapt to ever-changing circumstances, be on the lookout for unpredictable threats... in other words, they needed the very traits that make individuals with ADHD struggle in the modern world.
Emotional regulation takes energy. If an individual is expending a great deal of energy just attempting to get through each day because the world they live in is not one that their brain wiring is optimized for... they won't have the energy required to maintain good emotional regulation all the time. (I might add that plenty of individuals without ADHD also have a hard time regulating their emotions.)