Short answer:

Because they lived in Yiddish-speaking areas, doh.

Long answer:

The Yiddish language is indeed related to some German dialects currently spoken in what used to be Roman Germany but doesn't share the innovations that appeared in German after the 15th century. What might be the reason?

The Jews arrived in Germany together with the Romans but with the sudden appearance of Christians they started getting problems with antisemitism.

• in 1096 the First Crusade starts with a genocide on the Jews in Speyer, Worms and Mainz.

• in 1298 Rintfleisch burger movement is killing Jews and taking their property during a short war between the house of Naussau and the house of Habsburg for the German crown.

• in 1336-38 the Armleder movement is murdering Jews to take hold of their property again in Franconia, in Hessen and in Alsace; it's triggered by a noble who has seen an angel in his dreams.

• in 1349-51 the Jews are massacred in Speyer, Worms, Mainz, Koblenz, Köln and Trier in a Christian attempt to atone for the sins that have brought them the plague.

Inbetween you had smaller killing sessions, too. The Jews of Bacherach, for instance, have been murdered over the libel of having ritually cannibalized a Christian child for Passover. The child in question was made into a "Saint Werner". Until 1962 he was a patron saint of wine.

However: In Eastern Europe they were more than welcome for their urbanite know-how. The very first Polish coins have, for instance, been minted with Hebrew inscriptions.