Because he's young and 2 of the big four are likely to be gone while he's still playing.
Federer will retire in the next few years, and Nadals knees will catch up to him. Djokovic and Murray will be past their prime years while Del Potro is still in his prime years. There also doesn't seem to be any 90's players looking like they break will break through in the next couple of years
He is only 1 1/2 years younger than them and he turned pro the same year Murray did.
I'm not saying he won't ever win again though. He very well could, but the age thing won't be a factor.