Self-demagnetization
A magnetised medium builds an internal field that fights itself — the wall that stopped longitudinal recording.
The magnetisation in a medium sets up an internal demagnetising field pointing against the magnetisation itself. It is strongest right at the bit transitions, where two oppositely-magnetised regions meet. Pack the transitions closer and the neighbouring bits push harder on one another.
In longitudinal recording (LMR), where the magnetisation lies in the plane of the platter, this became a hard wall: bits began erasing each other as density approached a few hundred gigabits per square inch. Perpendicular recording (PMR), commercialised in 2005–2006, stood the magnetisation up out of the plane so that the demagnetising field stabilises dense transitions instead of smoothing them away. That change of geometry is what kept the hard disk growing.