I think it boils down to conflicts which does not involve UK or US, who have the biggest communities of gamers, don't get much interest and thus turnover.
I think that plays a part of it, but there is one HUGE except... Napoleonics. Granted you have Spain and Waterloo, but most big battles involve opponents such as Austria, Prussia and Russia. When I gamed Napoleonics in 15mm we had 3 Corps in each army on a ping pong table. The allies were 1 Corps of each of the above named countries. No British.
Once again I will say that I think one of the big reasons folks shy from investing heavily in the FPW is that you don't have the same "excitement" of other major conflicts. The Prussians kicked butt so badly and quickly during the unification of Germany that you almost don't have any "what if" scenarios. It's kinda like playing Sparta vs Athens over and over.
Nevertheless, I almost have my first 6 Pendraken French 10mm minis complete and they are looking awesome!