The most important reason for Asimov's Laws not being applied yet is how robots are being used in our real world. You don't arm a Reaper drone with a Hellfire missile or put a machine gun on a MAARS (Modular Advanced Armed Robotic System) not to cause humans to come to harm. That is the very point! The same goes to building a robot that takes any order from any human. Do I really want Osama Bin Laden to be able to order about my robot? And finally, the fact that robots can be sent out on dangerous missions to be "killed" is often the very rationale to using them. To give them a sense of "existence" and survival instinct would go against that rationale, as well as opens up potential scenarios from another science fiction series, the Terminator movies. The point here is that much of the funding for robotic research comes from the military, which is paying for robots that follow the very opposite of Asimov's laws. It explicitly wants robots that can kill, won't take orders from just any human, and don't care about their own lives.
io9 - The 3 Laws May Not Be Enough To Guide Robot Warriors - Wired for war