I think the change happened over time in a step-wise fashion.

Aluminum frames came on the scene. At first, they were lugged just like steel (ALAN, 1972). Then titanium came on - TIG (?) welded. Then aluminum went welded, again TIG welding. Once welding gained market acceptance from the consumer and was the production process on the floor in the bike factories, people began to weld steel, too. Welding a steel frame was possible ($) in part because you can TIG weld without the problems of old-fashioned arc welding (embrittlement, contamination).

Well, that's the way it seemed from my perspective. The industry insiders will know better than I.