This is because, following current best estimates, negative aerosol forcing has substantially offset the GHG - induced warming. (link.springer.com)
As I said to Andy Revkin (and he published on his blog), the additional decade of temperature data from 2000 onwards (even the AR4 estimates typically ignored the post-2000 years) can only work to reduce estimates of sensitivity, and that's before we even consider the reduction in estimates of negative aerosol forcing, and additional forcing from black carbon (the latter being very new, is not included in any calculations AIUI). (dotearth.blogs.nytimes.com)
The assessed best estimate was that the historical negative aerosol forcing (radiation and cloud effects, but not black carbon on snow / ice) was too strong by about 0.3 Wm - 2 in the models that included that effect, a conclusion very much in line with a prior publication on climate sensitivity by Otto et al. (2013). (realclimate.org)