This is to be fair similar to what happened at the 2011 Vuelta, everybody retro-justified it as being a horrible field which helped explain Cobo and Froome's prominence, but you also had Igor Antón who was one of the favourites coming in after crashing out of the lead in 2010, Scarponi who was at that point 2nd in the Giro (later upgraded to winner), Nibali as defending champion, Denis Menchov a two-time former winner of the race (at the time), and Joaquím Rodríguez, a top rider the previous year and in the top positions of the Giro, as well as Carlos Sastre's career swansong, but all of them underperformed massively or were simply not GC condition at that point in their career, leading to a very underwhelming GC mix.
Looking at the results sheet here would suggest this is a very disappointing field, but we've got Roglič, Thomas, Nibali and Geogeghan Hart as GT winners too, Porte, López, and several others, plus the likes of Kruijswijk too who have managed podiums at some point in their past, who don't appear in that top 10, while there's still another GT winner (Carapaz) and a couple of riders with GT podiums (Mas, Urán) in there. It's not as bad as it looks.