This is not complicated at all.
Marius Bakken startet with treshold training, had great success, knowledge got passed on to several others due to the same coach (Buraas 13.06, Sondre Moen 2.05). Then the ingebrigtsens made a template week with double treshold, same week in week out —> Success.
Now EVERYONE follows that formula to some degree. Its just a way better method of training. There has been some double treshold coaches in the US, but not as good/scientific. Most athletes have a lactate meter, and keep to the levels adviced. Simple and easy way to run many miles without overtraining, 100 miles a week is pretty normal.
Gotta remember that there is still way more athletes pursuing cross country skiing, way up to 25+ years of age. Success on track keeps some of these talents at the track, they’ve realised its possible to get to a world class level at running, which previously was a pipe dream.
You all know how these workout weeks are, still baffling that you keep doing your old tile training. You think american football guys train like they did in the 80s? :)