States, Russia and Germany have won Slams in the last 25 years (and plenty for US and Russia) despite tennis being a far less important game than in France (second most practiced sport in France behind Soccer, i repeat) so sorry but those countries's problems are far less important than France's problem (it just happens that they now know a small period without any outstanding player).
Italy??? Tennis is far less important than in France and it can be seen both in term of number of registered players and money invested in tennis.
Switzerland has won far more slams than France both on the men's and women's side.
Belgium as well on the women's side.
I don't think tennis is a bigger sport there than in France. Tennis is not ice hockey.