Because very few people know about it, as it has been covered up by the media. Heck, if a Iowa caucus voter didn't know Mayor Pete was gay until after she had voter for him, it's not surprising nobody knows about an obscure Bernie book from 1972.
If the Dims & media are really "scared to death" of Bernie winning the nomination, they can do a big part in stopping him by thoroughly vetting him. They need to give Bernie 92% negative coverage like they have given Trump for the last 3 years.