The fx are outdated and significantly behind in terms of performance. They were popular because they were inexpensive for the number of cores vs intel and many were on a 'moar cores' kick. Core count nor clock speed is the entire story, performance/power of the individual cores plays a big role (ipc performance). Fx was lacking and in many tasks was being beaten out by 3rd and 4th gen intel chips at the same price range despite having 1/2 to 1/3 fewer cores in just about all tasks. They were in dire need of a replacement to improve them a few years back and now amd has finally released ryzen.
Ryzen's addressed many of those issues, their boards include more modern/current features as well as moved to the ddr4 ram standard. They improved ipc quite a bit, bringing amd up to intel's ipc performance somewhere between 4th and 6th gen. Ryzen's clock speeds look slower compared to fx but because of ipc improvements similar to intel, they're actually faster than fx.
Long story short, fx weren't great gaming chips a couple years ago and even less so today with better options from both intel and amd. If someone already has an fx and it's fitting their needs then that's one thing, purposely building around fx isn't a good choice for a new pc build.