Someday,
good history, thanks. Those Yidden who did not see Yiddishkeit attractive, had a choice of commies/bund/zionists (2 types: anti-religious or religious). Simplification, of course.
Out of these, both zionist groups have descendants who are Jewish, and even many non-religious ones have religious or traditional ones. Bund/socialist types were wiped out by commies first, and remaining by Nazis. Commie types went thru same 2 types of gehinom, plus some became murderers themselves. The remnants who re-appeared in 1970s as “soviet Jews” were in much worse shape than descendants of Zionim, both physically and spiritually, with few exceptions of heroic baalei teshuva and dissidents among them.
As to who so many left religion at that time – it is both the lure of revolutionary times and, to a degree, failure of Jewish communities to have a strong alternative. One might argue that religious Zionists created an attractive alternative to those who would otherwise have left, or at least tried.