The advent of “strip cast” process versus the “ingot cast” (cupcake or book mold type casting), allowed to make alloys with higher homogeneity and far better microstructure - another reason for greater improvement in the unit magnetic properties.
The above improvement made possible, the use of NdFeB in many new devices specially those that required exposure to higher temperatures and higher demagnetizing forces. The NdFeB magnet users and manufacturers therefore fell heavily for magnets using progressively more and more heavy rare elements, as the end users designed their magnetic circuits, conservatively, around alloys which may had much higher coercive force than they really required. The cost of the heavy additive was reasonable enough to justify such change.