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That and increased clutter, and regulation:

Dr. Michael Potter, a professor and urban entomologist at the University of Kentucky, stated, “What made this problem pretty much go away for decades was the availability of very effective, long-lasting insecticides that were cheap, affordable and available not only to pest control companies but to consumers over the counter,” he said. Having conducted a study comparing older pesticides to those used today, he found that insecticides in two families, organophosphites and carbamates, are especially effective at eradicating bed bugs. While some of these compounds can be found in insect traps or spot treatments, most are no longer permitted for use by pest-control professionals in North America.

...carbamates, which include carbofuran, aldicarb (Temik), chlorfenapyr (Propoxur), carbaryl (Sevin), and bendiocarb (Ficam). Carbamates typically degrade faster than organochlorines, and some of them can be just as lethal, while others are seen as less toxic. The use of such is restricted by the EPA, and which has banned the highly toxic carbofuran, while the manufacturer of bendiocarb has voluntarily cancelled its registration, which can cost up to $100 million in order to obtain EPA approval.