| There are multiple aspects, their influence on the final manufacturing cost is the motivator. But it always has been so. What have changed are the relative costs of the individual materials versus their processing and even the alternatives.
So let's keep the acceptable quality level aspect aside for this moment (it is clear, lower quality requirements allow for lower cost).
What have changed was the cost of raw materials vs their processing. Decades ago mainly the raw copper was rather cheap, but the advanced thermally resistant insulation materials and processing of the advanced magnetic steels (high saturation flux, techniques to get thinner and more reliable insulation layers to prevent eddy currents,...) were rather expensive.
So it was cheaper to design the core for lower flux levels, so tolerate cheap core material, oversize the winding so it runs cooler, so no expensive high heat resistant insulation materials would be necessary and so on.
But in the meantime the cost of the copper had skyrocketed, while the advanced technologies for the high temperature tolerant insulation materials and way better performance core assembly had fallen, so the situation had changed a lot:
Now it becomes way cheaper to use high flux core steel, so you suffice with less turns. That means the wire becomes shorter. And for the same resistance the wire could be then thinner. Less turns and thinner wire means quite a lot of copper savings, the need for smaller winding window means smaller ballast size.
As the ballast get smaller, it's thermal resistance to ambient gets higher, so with the same losses it will operate at hotter. To prevent that, the losses would have to be lowered compare to the old design, so the savings won't be as high. But high temperature insulation materials and design the ballast to operate hotter, but as it becomes smaller, the reduced copper cost pays off well all the expensive insulation materials and steel processing.
With the magnetic the difference is not only the material properties alone, but as well the balancing between the core vs winding mass (smaller core means more turns, so more copper, less magnetic steel and vice versa)
But the thermal margin (that ends up as a lifetime) of that design is still the same as it was with the old one: The old operated at colder temperatures, but it's materials did not tolerate much more, so the temperature gap was not that high.
Today the operating temperature is higher, but because the materials could tolerate way more then before, the difference, so the temperature gap remains the same as with the older design, even when the resulting product is smaller and lighter.
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