Are you familiar with the phrase "Lügenpresse" or lying press?
It's a derogatory term used by the far-right or "alt-right" of mainstream media outlets. This stems from the notion that the media doesn't cover the same news they see in their social media bubble, therefore they think the mainstream media is intentionally covering things up and thus the conventional sources are not to be trusted. Social media does affect how we perceive reality, and if that perceived reality is different there is a conflict and you're more likely to trust your own bubble.
These shitty racist tabloid rags pander to those markets. Instead of some lonely individuals' conspiracy rant blogs that are easily dismissed by many based on their appearance alone, Breitbart and its ilk gives you the perception of a reputable news site. It is the same phenomenon as with someone wearing a white lab coat, it grants you instant authority. If it looks like news, it must be news, right?
These "news sites" are popular because their users use the sites not only as their primary source of news but many times as their only source of news. If it conforms to your reality, it must be true.