Just a few years ago, “pebble bed” reactors using thorium pellets was touted as the coming thing for generating electricity. Supposedly they would have allowed development of smaller scale reactors, allowing decentralization of power production, thus less losses to long distance transmission, much safer/simpler waste disposal, etc. Any idea why this idea has seemed to fade away?
Nope. You're confusing two topics. the "pebble bed" is a form of gas-cooled fast breeder reactor...still uses urani um 235 and 238 and makes plutonium. Stringent testing proved that the pyrolytic graphite "pebbles" didn't contain the fission products well enough, and shed a HIGHLY radioactive dust.
See Wikipedia https://en.wikipedia.org/wiki/Pebble-bed_reactor
Thorium also still uses U-235 (at least at the start), but does get rid of the U-238/plutonium problem. It is a type of molten-salt reactor.
see wikipedia https://en.wikipedia.org/wiki/Thorium-based_nuclear_power
There may exist a version of thorium that does use pebbles of some sort, but I've never seen any info for such a beast.