I assume (based, in particular, on the interview given by Gleikhengauz and Shcherbakova) that events unfolded something like this:
1. Medvedeva was Eteri's first figure skater, whom it was decided to seriously train to perform complex cascades and transfer important elements to the second half of the program. Then, after the Sochi Olympics, this was the most achievable path to success.
2. A little later, the preparation of Zagitova began, in which the same principle was brought to the absolute. That is, relatively speaking - Medvedev is the most advanced prototype, and Zagitova is an example of the same technology already brought to perfection.
3. Experiments with quads began in the 2016-2017 season. The first who successfully land such a jump was Anna Shcherbakova. This happened in April 2017. A little later, Sasha Trusova achieved the same.
4. Zhenya in April 2017 was already a two-time world champion, the next season is the Olympics. Theoretically, seeing the clear successes of Shcherbakova and Trusova, the coaches at that moment could have tried to transfer the new technique to Medvedeva and Zagitova, but did not do this. There are many reasons, but the main one, probably, is the well-known saying in Russia "The best is the enemy of the good." In other words, the study of quads could distract Zhenya and Alina from their "usual", but very succesfull programs, and no one guaranteed a successful transition to new jumps.
5. After the 2018 Olympics, Zagitova, according to Eteri, “relaxed” (I think there is some truth in this), and Zhenya left for Canada. With all due respect to Orser, he did not have the technology that just then, in 2018-2019, was being developed at Khrustalny. In addition, Zhenya's health problems were growing. In 2019, she reported that she was training 4S, but apparently it was too difficult to achieve this goal in those conditions - and then the coronavirus and so on. All in all, time was lost.
And in 2020, Eteri has two "advanced prototypes" at once, but of a new level, and Valieva - a model that was simply brought to perfection before our eyes. And even if Zhenya did not have her injuries, catching up with new stars would be an almost impossible feat for her. Meanwhile, the very idea of using quadruple jumps could well have appeared earlier, it was a matter of chance, in general. If Zhenya, for example, joined the experiments of Gleikhengauz and Dudakov right away, she could well get several quads, not only 4S.
