Mostly because what people refer to as "printing money" isn't really printing money, but more of a "reversible exchange of one class of assets for another class of assets of equal value". The central banks are not allowed to just create new money to give everyone working there a huge bonus, it always has to be swapped for a credible promise of future repayment. Banks take it up, since the rates are/were at near 0%, but people/businesses are not confident about their future money flows to take on new loans. Hence the new money just sits at banks and never reaches the real economy.
Helicopter money (1) does cause inflation, but the reason that the inflation has been stubbornly low for a decade is precisely because the central banks are forbidden from doing helicopter money