Galut Judaism is well-built on the principle of preservation, as well it should be. Without that built-in inertia (set to "grindingly slow"), things would have changes too quickly to maintain the system.
If you think about it, this was really beginning to happen in Eastern Europe before the Shoah. Rabbanim from there were making rather shocking halachic decisions, and despite Artscroll's attempts to change the historical record, very forward-thinking Yeshivot and rabbis were sprouting all across Europe, starting mostly in Western Europe but filtering their way east. Rav Kalman Chameides, the father of a member of mine from West Hartford, Dr. Leon Chameides, served as a rav in Katawice, Poland in the mainstream Orthodox shul there. Leon showed me some of the essays that he wrote in his local Jewish paper, and many of them were anything but "mainstream" by today's standards. (See this article - amazing!)
Which gedolim ended up in America? The most prominent leaders were also the very rabbis who were willing to leave Eastern Europe by choice. By nature that would also make them the most flexible. Rav Moshe Feinstein left Lituania in 1936 - well before the Holocaust. The Rav left Germany and followed his father as to America as well. He certainly could have found work somewhere in Germany, or Poland. Rav Shraga Feivel Mendlowitz came to the US in 1913! R' Yaakov Kaminetzky came in 1937 - not the best of times, but certainly before the worst of times. Rav Ruderman came to America in 1930 settling first in New Haven and then moving to Baltimore.
Can you imagine how many people thought that they were nuts to leave the safety and security of Europe for the "tarfut" of America?
It stands to reason that their halachic attitudes reflected their personalities - certainly adherents to tradition, but willing to adjust, change, and remold. It should come as no surprise that all of them sanctioned learning together with college study: Rav Ruderman in Ner Yisroel, Rav Moshe in Lower Manhattan (my father got semichah from Rav Moshe but eventually got a law degree), Rav Shraga Feivel was perfectly willing to open a college associated with Torah Vadaas. Many of Rav Moshe's piskei halachah (see Agunah, ger katan and many others) are certainly creative, if not revolutionary. But he had the broad shoulders to carry that great weight.
Of the group of big-time Roshei Yeshiva who moved to America, only R' Aharon Kotler came following the Holocaust after there was literally no choice but to get out or be killed. The article about Rav Hutner (who learned in Hevron and made his way to America as well) states:
He viewed secular studies as essential in learning a profession for people to support themselves by eventually going to college and becoming professionals. Together with the dean of the Yeshiva Torah Vodaas, Shraga Feivel Mendlowitz a charter to set up a combined yeshiva and college was obtained from the New York State Board of Regents. However, this plan was abandoned upon the insistence of Rabbi Aaron Kotler the anti-secular leader of the Lakewood yeshiva (Beth Medrash Gevoha), which would become the largest yeshiva of its kind in the United States, who wielded great influence and rabbinical power. In this and other matters Rabbi Hutner acquiesced to Rabbi Kotler.From the best of my understanding, Rav Aharon was also the one who was the strongest voice against change in America. To me, this makes sense. Rav Aharon was the most anti-change, as opposed to Rav Ruderman, the Rav, Rav Moshe - all of whom moved with the times.
But then again, which yeshiva was the most forward-thinking about community kollels, and spreading its message? You got it - Lakewood. They set the tone for chareidi America, clearly in the R' Aharon's mold. He set the tone for American Judaism, and Lakewood represents the greatest stronghold of Chareidi Orthodoxy in America.