Historically, why and how were the Literary Digest poll of 1936 and the Gallup Poll of 1948 flawed?

CNN: The Polls

In 1936, the Literary Digest, using a telephone and postcard poll, forecast a sweeping victory for Alfred Landon over Franklin Delano Roosevelt.

FORBES: Magazine Article

Seldom, if ever, said the Literary Digest in 1920, have the victors in a great war left the punishment of the defeated leaders in the hands of their own people.

NPR: Listening In On the Nuremberg Trials