Immigration to the United States was very different prior to the war and after it. In 1924, the US enacted the 1924 Immigration Act which put a limit on how many immigrants from each country could enter. This had a huge impact on Jews fleeing Nazi Germany starting in 1933. People wishing to emigrate from Germany to the US had to apply for one of 25,957 visas avaliable per year.
The immigration process was hard and expensive. There was a lot of paperwork needed, waiting, arrangements, and securing a visa before one could leave. The US was also dealing with the Great Depression, which also put a strain on potential immigrants. Getting to the US was a lot harder than getting to another European country. That's why so many Jews would flee to neighboring countries: Netherlands, France, Poland... and other countries like Canada, Australia, parts of Africa, China, all over the world.
There was also an anti-immigration sentiment in the US, people were afraid immigrants would steal their jobs, thought immigrants were spies. Many Americans did not support immigration, even if the people were refugees.
One example is the M.S. St. Louis, a ship with 900 Jewish refugees that sailed to Cuba. But only a handful of passengers could get off. The rest were stuck on the ship and eventually forced away from Cuba. There was a push to accept the refugees in the US, even at least the children by Eleanor Roosevelt. But the ship was forced back to Europe and the last moment, France, Belgium, Netherlands, and UK accepted a limited number of refugees each. But in the end, over half of those on the M.S. St. Louis perished in the Holocaust.
In 1933, about 565,000 Jews were in Germany. About 300,000 fled between 1933 and 1939. Only 37,000 German Jews remained in Germany by 1950. Many that survived immigrated to other countries between 1945 and 1950. Today, there are about 250,000 Jews in Germany.
Sources: 1924 US Immigration Act