1) The government sprays more money around in welfare and salaries than they raise through taxes
2) The government issues bonds
3) The central bank prints money to buy those bonds
It's equivalent to the central bank printing money to pay for welfare programs and salaries (which go to common people in the streets)
It's easy trot out the "but the bureaucracy!" argument.
Helicopter money is in-your-face money creation which brings out the gold bugs and inflation hyperventilators. QE as practiced is in comparison too mysterious for people to latch on to effectively.
Helicopter money was never really championed by anybody in the first place, so nobody really had to argue against it.
It's really unfortunate that a perfectly reasonable policy never got a fair chance. I do think it has a lot to do with ideological bias especially among central bankers, and with the fact that a lot of people on the left, who should naturally be in favour of such a policy, are too averse to studying economics and finance thoroughly enough to participate effectively in such discussions.