Why didn't other countries stop the holocaust?

There were many reasons. America was isolationist. Most countries didn't know exactly what was happening, and they certainly weren't aware of the numbers of casualties. A lot of countries didn't want to get involved in what could have been bigger than WWI (and was). The reason wasn't that they did not know, it was hard not to know the obvious, perhaps not to the full details, countries did not attack Germany until they were at risk.

Countries also were not ready to be involved, and believe it or not there was a lot of "anti-Semitic" feeling in all the countries. The U.S. did not get involved until they were bombed by the Japanese which at the time was an ally of Germany. Also the people and the government did not want to get involved because they were experiencing Great Depression also known as the dirty thirties.