All attempts to make a theory of quantum gravity are unfalsifiable, because the relevant experiments are far beyond our means, much further so than building a practical quantum computer. String theory benefited from multiple rounds of unexpectedly interesting mathematical discoveries, which fired up people’s hopes and kept the fires burning. None of the other assorted proposals (loop quantum gravity, asymptotic safety, …) got lucky like that. Moreover, there’s a case to be made that if you’re an orthodox quantum field theory researcher, any attempt you make to quantize gravity will end up a string theory. Roughly speaking, there’s no regime in which gravity is the only force that you need to consider, so to make any predictive statements about some quantum gravity effect, you need to understand all the physics that happens at energy levels in between “warm summer day” and “immediate aftermath of the Big Bang”. String theory was the only possibility that suggested there could be a way out.

You could say that this just goes to show that orthodox QFT specialists lack imagination. The pioneers of quantum theory devised it in order to explain hot gases in glass tubes. Why should their same notions about what it means to “quantize” also apply to space and time themselves? And maybe they don’t! But proposing an alternative to quantum mechanics, or a modification of quantum mechanics that works in all the circumstances where we have already confirmed quantum mechanics, is no easy task.

“Fundamental” physics had a period of great advances, from the 1890s with the discovery of X-rays and radioactivity through the early 1970s with the establishment of the Standard Model. From then, we’ve been in “the stall”, as barbecue folks say. The big accelerators have filled in the edges of the picture and confirmed some predictions from that era, like finding the top quark and the Higgs. But they have yet to deliver a sign of beyond-Standard-Model physics that holds up under scrutiny.