I look at this a bit differently.
I would say that especially smart or gifted people often find that school comes easy to them. So when other kids are learning study skills or endurance through hardship, they are sailing through without much effort.
This is all fine and good until they get to college, or life in general and run across a problem they don't immediately know the answer to. Because they never had to work hard to succeed in school, they don't know how to push themselves through the hard part to success. So they either learn that important skill late in life, or (I would say more likely) the give up and either label the task as "stupid" or decide that they must not be as smart as they thought they were.
"Smart" is a gift and it makes a lot of things easy. But it's not the only tool people need. I'd say if you could only be smart or only be tenacious, tenacious wins out every time.