There was a time when there was simply almost nothing else in America. So whoever was born in America, or had to flee to America (and needed a visa affidavit with a job offer) ended up there, regardless of whether they agreed with every aspect of their hashkafah. At that time, prior to Bernard Revel, RIETS was closer to being a regular Yeshiva. It would be inaccurate to say they ‘separated’ from the Yeshivish Velt, because they were לכתחילה catering to an American audience at a time when the mainstream Yeshiva world was still located in Europe. Post WWII, when thousands of survivors arrived in the US, regular Litvish Yeshivos were also rebuilt in America in a somewhat modified form, and RIETS itself remained more or less what it was beforehand. Once other options were available, parents and bochurim could choose where to go. It’s not like RIETS was once an integral part of the European Yeshiva Velt and then split off and went its own way…
