They most likely stopped making them because sales were not high and the have discontinued several FiSTās items due to the platform being considered dead here in the U.S. due to Ford.
Now you could argue that there is a used market but Mountuneās business model is based on ,modding newer production models in concert directly with Ford. Given that they are aware of two things one is FoRS owners have more disposable income in their eyes. Two is they are actively now making Ford Truck mods,
and Marketing those.
Three, there are a lot of competitors now selling parts for the FiST at much less expensive prices . Especially downpipes which always is a tread lightly sales item as they are not even legal in California or states that follow California Smog laws.
But even after that they have discontinued some of their bracing for our car as well which leads right back to my first point.
Sorry but thats the full explanation by an observer. lol