The article below claims that milder store-bought peppers are the result of growers using different varieties, which is driven by larger (factory) consumers of the peppers.
Ah that makes sense. Seems to be in large part because there are varieties that are lower heat but higher flavor, and ceteris paribus, getting away with less jalapeno for same or greater flavor saves money. I wonder if anyone's weighed the amount of actual jalapeno in a jar of tostito's queso. Probably like a quarter gram.