Inquisitive, using a judaicized version of non Jewish languages is a custom adopted by almost every community of jews in our galus. Your examples are a proof of this. It was and is a means of maintaining a separation between us and goyim(open pesukim – ve’avdil eschem min ha’amim – i have separated you from the nations…Hashem said it himself, not the “hasidic rabbis”) as well as an alternative to using lashon kodesh for mundane matters. It has an even more important role in today’s time, as we are more integrated due to necessity in the non jewish world than we were in Europe.
“For some reason” is not at all difficult to comprehend; as you mentioned, it was the norm in most Jewish communities for millenia. So the reason why chasidim speak differently than goyim isn’t a historical anomaly; what you might ask is why chasidim didn’t make a yiddish/ladino/judeo-farsi/judeu-arabic/judeo-aramaic out of English. The litvishe in Lakewood practically do, with vocabulary heavily influenced by yiddish/hebrew/aramaic. Chasidim would probably say that we’re not on the level of pre war jewry and we need to try our best in ikvesa demishicha to maintain whatever we can from our forebears. Halacha (especially the rema) constantly reminds us of our inferiority in Torah compared with previous generations. We don’t know how to identify kosher grasshoppers, we don’t know how to differentiate between “thick” and “thin” mixtures regarding taaruvos, we don’t know how to make soft matzos, we don’t (for the most part) know how to do nikkur, the list goes on and on. People who learn halacha aren’t bothered by the axiomatic principle of yeridas hadoros, while it is a foreign concept to those outside of the yeshivos.
The gedolim of our time(including my rebbe) have said that usually we go down in generations slowly, with the current generation still being able to conceptualize the previous one – after the loss ot the wonderous pre war yeshivos, the gedolim say that it wasn’t a yeridah, it was a nefilah – a cataclysmic fall, to the point where our best hope is to cling to whomever we have left from that time with a humble awareness of our meager stature.
From that perspective, it’s no wonder there aren’t any decidedly “American” minhagim (aside from pizza for melava malka and making krias hakesuba into a kibbud, but i digress), nor is there an American yiddish in the formal sense – we’re not forming “nusach america”, the way European jews 1000 years ago were able to do.