I'll try not to be redundant but here's my opinion :
In Gen1, Psychic type was predominant, making it the obvious answer to tear through teams.
In Gen4, stat distributions made Steel and Dragon being the right answer to destroy your opponents. Obviously, everyone (nearly !) used to getting them in their team, to the epitome of stupidity ; 4 Dragons / 2 Steels teams.
"Then why ?" FWG core is very potent with awesome synergies, as there are useful double-typing especially with water (Water/ground, Water/steel, Water/flying, namely Swampert, Empoleon, Gyarados (even if they are OU I'm demonstrating a general law, don't flame) BUT it is walled and laughed by Dragon/steel Pseudo-core.
This way, FWG couldn't be dominant in OU when Garchomp or Salamence, or even Latias were in the place.
With the ban of those named ones, we are seeing a massive rise of FWG core, with bulky teams using (Exemple) Heatran / Celebi / Suicune cores. It stills get a Steel in the name of Heatran, allowing it not to be blasted by Flygon, the remaining OU Dragon.
In UU, now. Steel is reduced to Registeel, which isn't an offensive threat at all, (Heatran is one, thanks to its secondary type), and Steelix. Steel is pretty defensive down here.
Dragon is reduced to the rare Altaria, which isn't really a threat else.
Obviously, there isn't a needing for countering those, because people don't even pseudo-core with this, as it's pure horse crap. (Individually they can be good, though)
So as I explained earlier, FWG core becomes the "easy" solution, allowing mass coverage and bulkiness.
For my part, opposing to More Cowbell, I love to build something around a core. You can't take mons one by one and just add them "to see". A starting core is a good option, and it's known that FWG is a nice starting core, so here we go.