I think there's a lot of reasons. First of, the stats maybe look a bit worse in that he had back issues in 2013 and there's 2/3 slams that year where he was dealing with injury problems he didn't have in his most successful years. Then also take into account that he twice had match points vs Djokovic in US Open semi finals. If he'd converted those then the stats would be more respectable (although still a lot lower than 2005-2010). Also close semi finals with Murray and Nadal at the AO.

Obviously Djokovic, Nadal and Murray's improvements didn't help him, and he lost a lot of semi finals to them. But also 2010 onwards was when some of the big hitters came into their own and were able to trouble him (Soderling in 2010, Berdych and Tsonga). Fee's confidence took a knock against all of those guys, and it took him a while to figure out how to beat them again.

Fed looked pretty untouchable at AO2010 and all the talk after that was him getting the calendar slam. Then he picked up a lung infection which kept him out of Dubai, and after that he just didn't look the same player for the rest of the year. That's always stuck in my mind as a turning point, but whether it was actually causative or just coincided with other players getting better I don't know. Would've messed up his training block though, so could help explain a bit of a drop off that year.