- Advances in rocket engine design & tech to enable deep throttling
- Control algorithms for propulsive landing maturing (Google "Lars Blackmore", "GFOLD", "Mars Landing", and work through the references)
- Forward thinking and risk-taking by SpaceX to further develop tech demonstrated by earlier efforts (DC-X, Mars Landing, etc.)
Modern simulation and sensor capabilities helped, but were not the major enabling factors.
This means that 3d-printed copper (alloy) is an amazing process and material for them. You can build the kind of structurally integrated cooling channels that the people building rockets in the 60's could only dream about, and it's not a gold-plated part that required a million labor hours to build, it's something you can just print overnight.