Most everyone has a microwave oven in their house. Its 2.45 GHz frequency gives it a long wavelength so it can cook your food by penetrating deeply. Obviously, cooking people is a bad idea so the microwave weapon they proposed had a much higher frequency and thus a shorter wavelength. It would only only penetrate a little, where our layer of skin would absorb the microwaves and turn them into heat.
Obviously feeling heat from an invisible ray freaks people out and it was terrifically effective. of the 11,000 volunteers just over a handful got hurt and that was due to user error. It's still in development today - the original prototype weighed 7.5 tons and the latest iteration by Raytheon can instead fit on a truck and fire a few hundred meters.
Why isn't it in use? It can't work in the rain yet, or dust. And it takes 16 hours to warm up. But it will be at somepoint, but don't worry. Like anything else in technology, it can be used to do bad things, like by leaders who spy on all their citizens, hack into the phones and emails of journalists and use government agencies to penalize political opponents - but that is just worrying about nothing. If you can't trust the government, who can you trust?
Pain ray makes your skin tingle by Dr. Karl S. Kruszelnicki, ABC Science
H/T Alex Berezow at Real Clear Science