Jalapenos have become victims of their own success.

They're so popular and mainstream now that buyers (especially food processors) have started demanding high levels of consistency in terms of spice; Tostitos and the like want to ensure that every batch of salsa and such they mass produce is going to have the same spice level, which is understandable. Unfortunately this drive for increased consistency has led to an overall weakening of flavor, similar to what happened to produce like tomatoes and apples once industrialized farming and food processing created demands for longer shelf lives, consistency in appearance, etc., and like tomatoes and apples, the jalapenos you buy at most stores nowadays are going to be much weaker in flavor than they were in the past.

Jalapenos not sourced from these industrial-oriented seed companies are going to vary much more in terms of spice level based on growing conditions, like temperature and rainfall. There even exists nowadays the "nadapeno," which has jalapeno flavor but minimal spice. You can also remove the seeds and veins of the pepper before eating it, which makes a huge difference in reducing spice levels. Picking them earlier and growing them in "ideal" conditions for lessened spice is trickier, but can also help.