SDN and NFV can be seen, in my opinion, as indicators of a systemic technology trend (other indicators are Cloud-Fog Computing, Cloud Networking, C-RAN, etc), which is going to impact the Telecommunications Industries: the trend is the "softwarization” of the Networks, in wide sense. Well...similarly "computerization" started eating several other Industries, since a few years.
In particular, in the SDN, the software network control is decoupled from the data forwarding plane (which can be widely identified as "hardware") and because of this, the hardware network infrastructure is decoupled from networking functions and business applications which could be, then, logically centralized (e.g. in the Cloud). SDN should not be confused with NFV, which is about virtualizing some network functions that, in turn, could be executed on standard hardware, and that could be moved and instantiated in various locations. In fact, ETSI argues that SDN and NFV could be seen as mutually beneficial but they are not dependent on each other: e.g., network functions can be virtualized and deployed without an SDN being required and vice-versa.
Defined like that, SDN and NFV are not new principles. They were already proposed and demonstrated since quite a few years (remember for example Active Networking). But why then are they so popular today, often discussed as the most advanced innovation opportunity for future Internet?
The reason stands in the novelty of the techno-economic landscape, I believe: today there are the conditions making the potential adoption of said principles really feasible at competitive costs and with high levels of performance. This is basically thanks to the IT progresses in terms of performance, the dramatic costs reductions and the network bandwidth availability.
It can be argued, maybe for the first time, that shortly it will be possible to “softwarize, or virtualize” any network functions and service spanning L2-L7, decoupling them from hardware. Imagine the middle-boxes, for example. Nevertheless all of it is not just technology innovation. The speed with which this profound innovation will really impact the Telecommunications is dependent on a number of other key factors, like business sustainability (what business models?), regulation...and Users's adoption.