As has been mentioned, the Jews of Poland-Lithuania (and later Russia, A-H, and Prussia following the Partitions of Poland), mostly lived isolated in their own villages rather than being present (albeit in enclaves) in cities and towns, like they were in most of the world. This meant that many did not learn the language of the land, and that those that did often kept Yiddish as part of a connection to their culture and families. On the other hand, Sephardic Jews kept Ladino despite being fairly well incorporated and assimilated otherwise, so this is a flawed theory. But let's roll with it because I can't think of a better one.
A less tolerant PLC could prevent Jews from dwelling like normal peasants, forcing their presence only in cities, thus causing them to all have to pick up Polish or Russian or whatever. For example, despite Bohemia being settled exclusively by Yiddish-speaking Jews, they quickly all spoke Czech and a Judeo-Czech dialect existed (this is the best-documented Knaanic language, and sometimes considered to be the definitive one).
Another possibility would be a strange combination of tolerance and intolerance by which Ashkenazi Jews were kept out of cities, while pre-existing Judeo-Slavic speaking populations were kept in them; this could prevent Yiddish from "out-competing" Knaanic even among not-originally-Ashkenazi Jews (as "Ashkenazi" properly refers to Jews descended from populations in Francia).
But it seems very much possible for Yiddish and Knaanic to coexist: in much of North Africa and the Balkans we see coexistence of two different Jewish dialects: Ladino alongside Judeo-Arabic in the former, and Ladino alongside Yiddish in the latter! Cosmopolitan cities like Venice and Amsterdam also often contained Jews of multiple origins and languages, which kept themselves distinctive despite intermarriage (Venice contained four different Jewish communities! Italian, Ashkenazi, and Sephardi, and Syrian). I suspect that Knaanic got outcompeted because of the sheer size of the Yiddish-speaking population.
Earlier and/or greater migration of Jews into the region probably helps this Knaanic stand up for itself. One good path is more Jews following Eastern Roman missionaries up into eastern Europe as the pagan Slavs become "civilized" by exposure to Jesus and Greek.
