Because the Quora Digest consists of user answers such as yours, and different subscribers to the digest receive different answers in the digest, which Quora thinks match their interests.
If you’re asking, why does Quora send out answers in the Digest at all… well, Chris Lynam’s answer to What is the Quora Digest? If I have a link to a story on Quora Digest sent by a friend, how can I read it?
First things first, the Quora Digest is a compilation of the most engaging answers on Quora. It’s also a way for Quora to nurture their audience, and utilize great content to power their email engagement.
Think of it like the Quora email version of a greatest hits album.
Or at least, that’s the intent. See also What’s your opinion of the range of material selected for Quora Digest? Should a wider range of topics be selected?