Its more likely to ask why wouldn't you want a smart tv. Otherwise its one more device, two more wires, one more power brick wasting energy, one HDMI port used and extra complexity for non nerdy gadget types.
The answer is because Smart TV UI's always suck (performance too) and that the tv manufacturers can be counted on to do stupid things no one ever asked for. Like including microphones and cameras (could easily be add-ons for the rare people who actually want it), and injecting ads into tv streams.
Thes extra gadgets only exist in the wake of the TV manufacturers ineptitude.