What an interesting thread...
It is interesting, in the last few years I've become a detergent hound of sorts. I really enjoy shopping for it, and buying it, and of course using it. In 2008, I remember a point where I had to rinse a bottle of liquid (to get use of the several tablespoons in the bottom) and mix it with a half scoop of powder just to get a load of laundry washed, then I had to go to the store in a major thunderstorm to buy something else. Today? Not quite the same - I have enough detergent to last several (ok, many) years. I've been giving it to family, co-workers, friends, and charity on a regular basis trying to thin it out.
I too prefer powders, at least currently. This thread has touched on several interesting topics, so I'll try to add what I can where it might be helpful.
First, there are so many detergents available overall, that if a U.S. store had EVERYTHING that they could, they would far exceed their space allocation and we as consumers would probably be overwhelmed to the point of indecision. I remember as a kid when the detergent aisle in the grocery store seemed to take up the entire aisle, end to end. Not anymore, BUT, stores were smaller then and thus the aisles were shorter, AND detergent packages were bulkier. This was when we had one version of Tide, one All, one Gain, etc etc. Thus stores could carry most of the market offerings, but even then one chain usually had a brand or two that another didn't, or a size that another didn't.
It sounds like consumers in the U.S. and Western Europe are mostly liquid buyers now. Liquids are easier to measure, less likely to spill, and easier to carry. Even P&G will tell you that powders are better for certain stains, and liquids for others, but consumers are predominantly buying liquids, thus this is what stores are stocking.
If you want some fun, check out Tide's website, and that of Gain, Era, Cheer, All, Surf, Wisk, etc. You can learn all about the total of what's available from these brands. Most stores don't have half the available products from prolific brands like Tide and Gain. Odd that yesterday I found a type of Tide in a grocery here that is not yet on Tide's website -- Liquid Tide with Downy in Downy Unstoppables scent.
On Walmart - typically in this area, Walmart is one of the least costly places to buy detergent. Their shelves get picked-over because they sell so much. They do their re-stocking at night mostly when shopper volume drops off. They also seem to have a very large selection including a decent supply of powders. I am not a Walmart fan so I seldom visit there.
My favorite grocery store here is a bit unusual in that it has a plentiful powder supply still. Tide (in two scents), Tide w/Bleach, Tide HE, five varieties of Gain powder, two Surfs, two Fabs, a couple Arm & Hammer, Purex, Trend, all this in more than one size. I've probably forgotten others as well. The chain is called Bi-Lo. Another local, Harris Teeter, has about three total items of powder and is almost insulting.
Gain has just started marketing their powders at a new lower price. This will be interesting -- powder is already more economical on a per load basis typically as a 40-load box of powder sells here for about the same price as a 32 load bottle of liquid. P&G probably has under-utilized powder processing equipment they want to ramp up.
I agree with the comments about local stores, mom and pop shops, etc. They often have a brand or two, or a different variety than what can be found elsewhere. I am very cautious of Dollar General however. Their gimmick is to sell the 40 ounce bottle of the same thing everyone else is selling in 50 ounce, or the 75 ounce instead of the 100. The bottles look the same, but the shelf prices are lower, thus the consumer thinks they are getting a bargain, when in reality they are paying more on a per load basis. Recently while at the Bi-Lo, I was looking at the multitude of Tide, and a lady walked by, turned, looked at me, and said "General Dollar, General Dollar!". I said "Pardon me?" and she said "General Dollar! Much cheaper -- don't buy that here". I laughed...
One final thought for the author of this thread (mrwash) - if you want to tell us where you'll be visiting in the future, some of the members in the know with that area can suggest stores to visit that will be worth your time. I suspect the answer will be different for each major city that you visit.
Gordon