I'd say it's a feedback loop; the effect feeds back into the cause, sending Kodachrome sales into a downward spiral. This also makes it difficult to say what's the cause and what's the effect, since both factors function as both cause and effect.
Also, even a few decades ago, Kodachrome had to be sent out to one of a handful of facilities for processing, whereas E-6 (and earlier Ektachrome processes) were more easily done locally. This would give a speed advantage to Ektachrome -- not as great an advantage as exists today, but a real one nonetheless. Once some other factor (improvements in E-6, decline in film sales generally, UFO thought-control rays, or whatever) causes a dip in Kodachrome sales, the aforementioned feedback loop would take over.