EDIT: This is about the Jewish community in the United States. Please give answers about American Jewry as I understand that the dynamic is very different in Israel.
I know I have better sources for this from an American Jewish History class I took, but for now I found this source: https://www.interfaithfamily.com/religious_life/spirituality/the_case_of_the_missing_sav/
It seems like it can be attributed to several factors, including: that it was easier to find Israeli teachers to teach Hebrew to Jewish students, following the lead of the accepted practice in Israel, and that it felt more "modern" to use the Sephardic pronunciation that that of Ashkenazi/European Jews.
"Hebrew in America," edited by Alan Mintz looks like a good resource, but only parts of it are available as an eBook.