He failed to ruthlessly dominate his generation like Federer did. Specifically Wawrinka and Murray beat him on their way to 5/6 of their slams, 4 times in the finals. Federer only lost once to someone in his generation (Safin '05 AO) and ruthlessly dominated the rest of his generation (roddick and Hewitt). If djokovic had only lost once to Stan and always beat Murray, he'd be sitting on 16 slams with a great chance at getting to 18 and they'd both be one slam wonders. I don't think federer's generation was any weaker than djokovic's (minus Rafa), Federer simply was too consistent in his prime to ever lose to lesser talents.