It was marketing. During the early 1970s bicycle boom many people were transitioning from upright handlebars to the trendier dropped bars. Stem shifters (and brake safety levers) allowed people to ride on top of the bars in a position similar to that offered by upright handlebars.
For a novice, the shifting process of a derailleur was quite complex, involving multiple new operations, Though it seems quite natural to us, it provides a lot of anxiety for newcomers. Eliminating the need to ride on the drops and bend down, to reach a gear shift lever simply made the transition easier and a bit safer, in that stem shifters still offered enough upwards peripheral vision to see the road ahead of you, even if you were looking at the shifters.
The Europeans, who grew up on derailleurs and dropped bars were late to catch onto the stem shifter option, which was originally found primarily on Japanese built models. It was another clever way to tailor the models to the American marketplace, to gain acceptance. I'm sure manufacturers would have preferred to offer down tube shifters, as they are slightly less expensive, but stem shifters clearly appealed to the buyers of entry level models, who were typically inexperienced with derailleurs.