I'm not sure the premise of your question is correct. This website catalogs classic composers I suggest you sort by country and then also by popularity. I think you'll find that famous and great composers aren't necessarily limited by geography.

I can tell you've been listening to the modern "Canon of Classical Music" a lot! You can get this impression of German/Russian dominance from what they constantly play on NPR but it's not quite true historically.

During the baroque period your most famous and successful composers would be mostly Italian, with some Germans who did get very successful like Handel and Hasse. However, both of those men were really very Italianate composers, they studied music in Italy and composed for Italian singers. There were also successful composers in France and other places in Europe, but the composers who were considered good enough to be "exported" to other countries were usually Italian or Italianate.

In the classical period Italian hegemony starts to lax and you see more successful composers from German-speaking areas, in particular Vienna. However many of these composers are still pretty Italianate, like Mozart, or Gluck, they composed for Italian poetry and studied Italian methods. They also still work with a lot of Italian musicians and singers.

In the romantic period things really open up, especially for Russia. Russia had quite limited access to the European music scene in the 1700s and 1800s due to geography and a few political things, but they really joined the party in the romantic period. Look at the boom in Russian composers in the romantic period just on this list. French composers start to get more of their work popular outside of just France at this time as well. Italy is going through some pretty rough times in the 1800s, so while it is still actively contributing to the music in Europe, it's got some other stuff to work through at that time, so the other countries get a little more space to jump in.

So yeah, you can get this impression from the symphony circuit these days but art music wasn't really dominated by Germans and Russians. My explanation is pretty back of the napkin here (I just hand-waved the Napoleonic Wars and Italian Unification as "pretty rough times" which I am a little embarrassed about) so let me know if I can expand on something.