Why is it that Russian Jews spoke Yiddish and not a Slavic language?
Because they came from Germany long ago. Luther and his German followers weren't too keen on the Jews. There is a touch of Slavic influence in Yiddish, but it's impressive how much the Jews of Eastern Europe avoided taking on the language of those around them. Yiddish was my grandmother's first language. I'm sad that I didn't pick up much from her. She and her friends would speak it when they didn't want the children to know what they were saying.