The doors to immigration were closed to mass immigration from the mid-1930s and on. No country, not British Palestine and not the United States, were allowing mass immigration. Even if the Jews wanted to, they couldn’t leave. Before the run-up to WWII emigration was a bit easier but no one was a prophet to know that the worst genocide against Jews was coming up.
Furthermore, not only is it true that a poor Jew is more likely to remain frum than a rich Jew who has many more opportunities and available taivos to go off (and if that is what it takes to keep them frum it is better to be a poor frum Jew than become a rich secular Jew), but even more importantly pre-WWII most Jews who came to America from Europe threw their Talis and Tefilin into the Hudson as they passed the Statue of Liberty. (Figuratively, not literally. The point being that most of them — and especially their children — discarded their Judaism once in America.) So the rabbonim, wisely (not knowing in advance that genocide was to soon occur) advised folks to stay in Europe where they had been living for 2000 years.
levlalev: If you want to know what kind of provocateur Lipman is, see this video of him: