Well that is probably not the only reason why he dropped in ranking since that time. There are injuries, mental aspect of the tennis grind. Not everybody can handle the pace of ATP tour. I remember Bellucci, he was so close at snapping Nole's unbeaten streak in 2011, I think in Madrid, he was up a set and a break (I believe) and couldn't keep that up, got injured and asked for a MTO and from then on Nole won easily.
The development of a tennis player is just like everything in life, your own work for example. You have to be inventive, ambitious in order to prosper; otherwise you are just where you are without any improvements and possibly others pass you by as well.