Two or three reasons. The first being Israel is just that obvious a choice for a Jewish nation. It's really their only place they can claim historically. Because of which all Jews would gladly take the chance to settle down in Israel, 'land of their forefathers' and such. And except for the ultra-orthodox, many enjoy the chance to do so.

Then, there was already an attempt underway by the nationalist movement now known as Zionism, a secular group that sought to escape persecution of the Jewish ethnic groups of Europe by forging their own nation, that promoted immigration and settlement by Jews in the Levant. It was rather minor until WWII...


And then came the decision about what to do with all the displaced Jews and survivors of the Holocaust. We all know Hitler was the devil incarnate with a funny mustache, but many don't realize that most of Europe thought along the same lines as the Germans about the Jews - just not to the general extreme the Nazis believed. This includes Great Britain, who rather than take in any large number of Jewish refugees, started to send them down to Israel, their area of control according to UN mandate after WWI.


Those that didn't decide to go to Britain, and thus Israel, moved to the United States. It's why the vast majority of todays Jews are either Israeli or American. The rest are Russian and French, then the numbers dwindle quickly.