I think much of the prevalence of Germans in classical music was that until the 19th century, Germany was divided up into many different countries, each with their own court, and with their own court musicians. That simply gave more job opportunities for composers, and more people would try to compose. Even if you were not a Bach or Beethoven, you were more likely to make a living as a composer in Germany for one of the local kingdom courts, so you were more likely to try composing classical music.
In France and England, there was only one royal court, so there were just fewer opportunities for composers.
Same thing for Italians and Opera. Opera was very popular in Italy, and there were more opportunities to compose for an opera, than in England. Hence, you see a lot more famous Italian composer of operas than you do English ones.
Keep in mind, that for every genius, there a lot more merely talented people. If only the very best can make a living, that might discourage others from entering the field in the first place. For example, Holland and the low countries produced a lot of famous painters around the time of Rembrandt. At that time, a typical portrait painter could make as much as good carpenter, so even if you weren't a genius, you could make a good living. But nowdays, most painters don't make much money. A few lucky ones can make a lot, but most starve. Someone who might be tempted to became a painter if he was certain to at least make a living, might not try if he knew the odds were that he couldn't.