Firstly, it was considerably easier to do it on the moon. Low orbital speed, 1/6 the gravity, no air resistance on descent, very light lander (as it wasn't pushing 40+tons of second stage to orbit).
Secondly, this stage was doing Mach 8 to 10 at about 80km altitude when it separated from the second stage. They did an extra burn that briefly popped it out of the atmosphere, reversed its course, then did a hypersonic re-entry tail first and (nearly) landed on a 50x60m barge.
Nobody has done that before. Not the guys with the shuttle SRB's, they just fell back to earth (and were strong enough to withstand the tumbling in the atmosphere, being SRBs). Not Boeing with it's dinky little hops of 10,000 feet in a continuously-stable attitude at subsonic speeds. Nobody has gotten this far before with the return of the first stage of a liquid-fuelled booster. Seeing as those things are enormously complex and very expensive, it'd be great to get one back in one piece to use again.