Jonathan Ladd explains why:
Party polarization has raised the stakes in elections. And polarization combined with the growth of partisan media options has created an incentive for party leaders and activists to discredit the mainstream media among their supporters. Party leaders convince their partisans in the mass public to resist informative messages from the mainstream media and ideologically hostile outlets, and instead rely more on ideologically friendly new outlets. In doing this, they can help to inoculate their supporters against voting for the other side.
The trouble with this trend:
Political scientists have documented the tendency of people from different parties to have perceptions of reality that reflect their partisanship. Put simply, when a Democrat is president, Democrats tend to think that national conditions are better than Republicans do, and vice versa. I find that this trend is much larger among those who distrust the institutional media.