Why did the digest miss the current outcome in 1936?

The Digest (Literary Digest) committed a grave error in conducting the polls for the 1936 presidential elections. The digest was able to correctly predicted the outcome of the past 5 elections. However, during the 1936 election, the sample group they polled were not representative of the general population. The means they used which were telephone and car registrations. The population represented by these means were the wealthy ones. The wealthy ones were also staunch detractors of Roosevelt at that time because of they do not approve of his programs for the poor that will greatly affect the rich. And so, the outcome of the poll was a biased one where Landon got 57.1% of the votes and Roosevelt got 42.9% of the 2 million votes. During the actual election, Roosevelt won with 60.8% of the votes. A very obvious margin of discrepancy was shown between the polls and the actual electorate votes.