In people with ADHD/ADD, the prefrontal cortex typically is slower to develop than in those without the disorder. That area of the brain is responsible for executive functioning (which in turn related to impulsivity, decision making, planning, etc.)

I wouldn't say that ADHD/ADD causes low emotional intelligence. I actually think the sensitivity that comes with ADHD/ADD can lead to high emotional intelligence as many tend to be hyperaware of those around them. I would say that ADHD/ADD may hinder one's social skills (due to the lagging prefrontal cortex) and that impulsive behavior/being prone to outburst/sensitivity might appear as though the person has low emotional intelligence.