I believe a lot of it comes down to what I always say, which is only you can know what works well in your own water!
Where I live, liquids rule....but that's largely because launderers try the powders that are still available (Tide, Gain, etc., and they suds up like mad and don't rinse clean in soft water, like they can in hard-water areas.
Also, powders are not aggressively discounted in supermarkets the way liquids are, and there's only so much you can do to a cardboard box to make it look visually appealing.
It seems everyone and their brother's plumber has advised them, "powders don't dissolve", and that they have a bad effect on the innards of the machine (for the most part, I disagree).
One product that is wildly popular here is the Kirkland 300 oz. liquid Tide knockoff that is sold in Costco stores (there is also an All Free and Clear knockoff in the same size). I've used it myself, and know it's converted a lot of Tide purists that balked at P&G's most recent price hikes on their branded detergents.
In New Jersey, many of us have damp (if not outright wet) basements, and the powders, especially the less expensive ones, tend to clump up in the box. This is not a new phenomenon, but today's consumer is very convenience-oriented, and liquids in bottles solve that problem easily.
Lastly, many powders (Purex, Surf, and All in particular) don't dissolve well (or at all) in our soft water.