litvish, a lot of answers to your question can come from learning history- I’ll present some very simplistic generalizations to explain:
when Jews were in a comfortable galus, with lots of materialism, and earned high status among the goyim, who were also cultured, they were more likely to assimilate- that applies to Germany during the enlightenment, and also to Spain during the Inquisition- when faced with the choice of death, expulsion, or conversion, many chose to convert- either truly or in pretense, since it was very hard to give up what they had, and leaving meant a very unsafe, uncertain future. The fate of those who choose expulsion was not pretty. Within a few generations, even those who initially converted in pretense were lost to klal yisroel. After that, Yotzei Sefard did not achieve the sort of status they had in Spain in their new countries, the goyim around them kept them very separate, and were not exactly cultured, which could explain the lack of assimilation. Jews of Eastern Europe who were kept separate from the goyim, who lived lives of severe deprivation and who would never admired the goyim around them, chose to die or flee (when possible) rather than convert. That shifted dramatically in Communist Russia, of course, which may be the most dramatic example of a whole population of Jews losing their Yiddishkeit, except for a few yehidim. So it has nothing to do with ashkenaz vs sefard, but rather with the circumstance of the galus.
Interestingly, if you take 2 hypothetical Russian brothers of early 20th century. One stayed in Russia, one left to America. Look at the grandkids or great-grandkids of those brothers- say both made aliya in the 1990s. Standing in line at passport control at the airport, you will definitely be able to tell them apart, even though as cousins, they share some genetic background. Those who grew up in Russia will look Russian, those in America, will no longer have those Russian features. Environment, diet have a lot to say about how we look, as does who the brothers and their children married.