There are two reasons. One is that in the fixed mindset, great geniuses are not supposed to need it. So just needing it casts shadow on your ability. The second is that, as Nadja suggests, it robs you of all your excuses, Without effort, you can always say, "I could have been [fill in the blank]." But once you try, you can't say that anymore. Someone once said to me, "I could have been Yo-Yo Ma." If she had really tried for it, she wouldn't have been able to say that.
Mindset, Carol S.Dweck