Magic: The Gathering was off to a hot pace in the mid-1990s. The success of Alpha/Beta Limited and the early expansions pushed the limit for Wizards of the Coast's production with packs and boxes or cards selling out regularly. It got to the point where they didn't even try to advertise the game all that much and it still sold like hotcakes.
With 1994 proving to be one of the most stressful on record for the game because of so many orders and sellouts that year, WotC decided to do something about it: They ordered the largest print run for a Magic set ever to date.
Limited had a print run of roughly ten million cards (2.6M for Alpha and 7.3M for Beta). Unlimited had a print run of some 35 million. Arabian Nights was at five million, Antiquities 15 million, and Legends settled in around 35 million. When Fallen Empires was being planned, WotC decided to up the print run. A lot. When all was said and done, it was estimated that there were 375 million cards printed.
Part of the reason for the massive print run as the game's massive popularity, but Fallen Empires was also made to include the same card featuring different artwork on multiple commons (such as the case with Hymn to Tourach, which featured artwork from Susan Van Camp, Liz Danforth, Quinton Hoover, and Scott Kirschner). Even with the drastically increased print run, Wizards of the Coast still figured to sell through the lot fairly fast.
Despite estimates originally showing Fallen Empires selling out in a few months, the set sat on store shelves for years. And it's not that the set itself was terrible (it wasn't great, but that's besides the point), but the total cost of it all just didn't work out at all for Wizards or for the stores who ordered cases upon cases of the set. Printing that many more cards cost a fair bit, as did price reductions tell sell all the excess cards. And when no more could be sold, WotC had to warehouse all the millions of extra, un-shipped cards, adding to costs even more.
It was such a loss that it caused WotC to rethink it's printing amount for all runs in the future. The Wizards sales team came up with a printing formula in the aftermath and finally figured out a system on how many to print in comparison to sales, with retailers also figuring out how many were generally needed.
Today it's a well-oiled machine, with only small hiccups like the COVID-19 pandemic and other events out of WotC's hands on how card sales will turn out. Fallen Empires has been the warning ground for not only Wizards, but all card game makers. Figuring out how many cards needed beforehand, it turns out, is pretty important.
Still, Magic: The Gathering got past the market saturation level many fads never can get past and came back with a stronger strategy.
Even today, for such an early expansion, booster packs of Fallen Empires aren't nearly as expensive as other sets from its era. In fact, while packs from sets like The Dark and Ice Age (the two sets that came out before and after it, respectively) go for big bucks, Fallen Empires packs can be obtained for around $20 or so.