Why did flying become so safe?

Flying became so safe because the industry treats every accident as a lesson and builds the lesson into aircraft, training and rules. Aviation safety is the study and practice of managing risks in aviation, through research, training, and safer aircraft and infrastructure. According to figures summarised in Wikipedia's article on aviation safety, fatal accidents per million flights fell 12-fold between 1970 and 2019, from 6.35 to 0.51. The fatal accident rate fell from 3.0 per million flights for the first generation of jets to 0.1 for the latest.

Part of that is technology. Fly-by-wire from 1988 enabled flight envelope protection to reduce loss-of-control accidents. Part is the flight recorder, popularly called the "black box", though the devices are required to be bright orange. The flight data recorder logs dozens of parameters several times a second, and the cockpit voice recorder keeps the sounds in the cockpit, including the pilots' conversation, so investigators can find out what really happened.

The idea nearly did not get built. In 1953 the Australian scientist David Warren conceived a device that would record both instrument readings and cockpit voices. Aviation authorities around the world were largely uninterested at first, until Sir Robert Hardingham of the British Air Registration Board visited his laboratory in 1958.

And part is people. After the Tenerife collision, cockpit procedures were reviewed, crew resource management became a fundamental part of airline pilots' training, and the captain is no longer considered infallible. In the United States, from 2000 to 2010, driving was about 750 times deadlier per mile than flying in a commercial airplane. Counted per journey rather than per mile, though, the Economist finds cars safer than planes, so the answer depends on how risk is measured.