It's the lack of flow.

When I'm good at something, I can switch my brain off (even for mental tasks like programming; it's weird how ADHD works) and happily do it for hours.

When I'm working on something I'm not good at or am new to, I need to stop every few minutes to think or research and that gives my ADHD brain an opportunity to attack.

When I'm medicated, I can maintain that flow state with nearly any task - just with zero control over which task gets priority.