Most people who encounter Yiddish outside of the religious Jewish community encounter Western Yiddish. Most people who learn Yiddish as a first language today would be speaking Eastern Yiddish (think Ukraine, Belarus, Lithuania, Russia), which has more obviously Slavic elements.

The standard historical explanation for this is that Yiddish began as a Germanic language, and the Eastern dialects developed after Jews were expelled from Germany in the 14th century and migrated East. There were already Jews living there, but the population grew tremendously with the migration.

Paul Wexler's thesis is that the Eastern dialects of Yiddish actually independently descend from early Jewish Slavic languages (e.g. https://en.wikipedia.org/wiki/Knaanic_language), and the Western dialects are derived from the Eastern ones.