because most electricity comes from burning coal which is worse than burning oil when it comes to CO2 output, although most hybrids tend to be a lot more efficient (lighter, more aerodynamic) than a heavy duty truck (I'd imagine a hybrid truck could easily be worse in terms of CO2 output, and is a reason we haven't seen them yet. IIRC, Toyota will finally release a hybrid truck in 2016, albeit only a light duty Tacoma)
electric vehicles are only as green as the electric plants that produce most of their energy, the primary reason we've been pushing for them is because we have hundreds of years for coal reserves but oil could dry up (at least in terms of economic viability for use of just burning it) within a generation. Also, the US has a ton of coal but relatively little oil, so there is political/economic strategy to promoting vehicles that rely less on oil (energy independence).
the whole notion of driving a Prius (or any electric vehicle largely powered by the grid) because its better for the environment is ignorant; its better because it doesn't rely as much on one of the rarest and costliest forms of fossil fuels