It takes time and a bit of effort for a person to cut out a ballot from a newspaper, answer it, put it into an envelope, and mail it to the newspaper. The results from this type of survey will be quite biased. Only those with strong opinions, either positive or negative, about the topic will go through that much trouble to reply.
Work Step by Step
Additionally, "paper" newspaper readership is biased. Younger people are not as likely to read a paper copy of a newspaper as are older individuals.