Well, see, back in the 1980s, an agri hybridist was experimenting with making milder jalapenos to see if it could be done and ended up creating a Frankenpepper. Then the thought was, hey, doing that could increase the market for the pepper. They were just getting popular---back then, no, not every AM-PM Mini-Mart, 7-11, Maverick, and whatever convenience store had jalapeno rings---and with so many people talking about them, it was thought that mass-growing a milder jalapeno would increase market share. Well, guess what? It worked. The jalapeno became more popular than anyone dreamed. But the problem was, this Frankenpepper got into the general cultivar gene pool and now jalapenos are getting milder except for those people are breeding to make hotter. I seem to recall the Frankenpepper jalapeno was made by splicing in some bell pepper stock, and that's how they made it milder.
The Frankenpepper is by no means the end of the jalapeno. But most of the ones you buy in the store are going to be the milder Frankenpeppers. Unless you live in the Southwest. Here, our jalapenos are imported from Mexico and they are nice and hot. It's a reverse thing here, see. People here won't buy them if they're not hot. Now, yes, a place like a supermarket here, they might be the Frankenpepper because they'e buying from McProduce McDistributor. But the Hispanic mercados here, you will get real jalapenos. You should also look to your Community Supported Agriculture farmers markets / community markets as the growers there will be growing proper jalapenos and not the Frankenpepper. The Frankenpepper is the seed stock sold for commercial growers and goes by several different names. It is correct that many of the commercial cultivars are/were selected for big crop yeilds, but of those, they also selected for the Frankenpepper so the jalapenos would not be too hot. They are, after all, never sure what purpose the jalapenos are going for be it pickling, salsa, fresh, processed food, or nacho rings at the 7-11. So, they opt for Frankenpepper to make sure no one gets burned, so to speak.
The solution is this: Grow your own, go to CSA farmer's markets, and use ethnic grocers whenever possible. It's no guarantee that ethnic greengrocers won't buy Frankenpeppers, but they're less likely to as their customers will often demand HOT peppers. Look to Southeast Asian, Indian, Pakistani, African, Latin American / Hispanic, and Caribbean grocers for good, hot chiles. You'll also find some hot chiles at some Middle Eastern and North African grocers, in particular those that cater to people from Yemen, Tunisia, and Morocco.
Also, canned or pickled jalapenos from any "brand name" will usually be Frankenpeppers. They're selling to everyone and can't afford stuff not getting sold because some people found it "too hot". Look for small-brand pickles from people who offer real hot jalapenos. They're also amazingly easy to pickle yourself. And when you pickle jalapenos yourself, you can pickle them in apple cider vinegar instead of distilled vinegar. MUCH, MUCH better pickle and the vinegar is great to use on greens. I talk about this stuff all the time on my blog. Check me out sometime.