There's a lot of factors regarding why a deck will get that slight warp to it - it's not an exact science and no one's done careful studies of it, though heavier stocks like casino-grade Bees will warp less than Bicycles or similarly thin stocks. The laminate on the surface of the paper could be a factor as well, in that it seals the paper and doesn't allow for the easy transfer of air and moisture except at the card's edge where it's been cut.
If you have a deck that's not as flat as you'd like it, the most common cure for this is storing it in a Porper clip or a similarly-made deck clip that applies pressure to the front and back of the deck. I also have a deck press - these were common over a century ago but are more scarce now, though magic shops will carry them for magicians to take the warp out of expensive gaffed/trick decks. Good deck presses are hard to find - the best are the old-school models that can hold a dozen or so decks at once, with a single central screw mechanism used to apply uniform pressure across the card surface of all the cards at once. (Mine only holds two, and has screws at each corner instead of a single, large screw to provide even pressure.) It can probably apply more force than a Porper clip, but the cards have to be removed from the boxes - it will crush and destroy the boxes.
The reason they were popular in the latter part of the 19th century was that playing cards were still rather expensive, when you factor in what people were earning in pay those days. In small-town saloons throughout the Western US, often players couldn't afford their own cards and wouldn't trust someone else, so the barkeep would have a supply of decks on hand for patrons to use. At the end of the night, the barkeep would gather up the decks (without boxes - some companies were still wrapping their decks in paper, and again, the press would only crush them) and place them in the press, using wooden separators to keep them apart from each other. The pressure kept the cards flat and protected them against excess moisture leeching into the paper, thus helping them last longer.
As cards got cheaper, deck presses fell out of favor - when a deck went bad, you simply bought a new one. They were still pretty useful in very humid climates, but even there the cost of replacement cards just got cheap to the point that almost no one bothered using presses any more. It's a relatively recent phenomenon that there are such expensive decks of cards becoming more widely available, so perhaps it's time for a revival of the gadget - but as of yet, that hasn't happened. I wish it would already - the cost of a single deck press to protect a dozen decks would probably be less than the cost of a dozen Porper clips to do the same job. Last time I checked, a Porper is around $25 for the name-brand item rather than a knockoff, so if a well-made one-dozen deck press is significantly less than $300, it could be a popular item for collectors, cardists and magicians.