as approaches that don't work get abandoned. Also, with automation, various approaches that have always worked but were expensive to implement can now be used in inexpensive movements.
However, there is a down side: modern movements are made using materials that can be worked by machines, and watchmakers have told me that the old movements use tougher alloys and also that the parts are more thoroughly hardened.
One other thought: most old movements are designed to work even when everything is not "perfect," as that was necessary in days of hand manufacturing. Now, they may not be able to tolerate problems as much because they rely on very tight tolerances. Of course, in these days of automation and tight quality control, the incidence of problems is reduced, so the modern approach makes sense for modern movements.