There's no vast left- or right-wing media conspiracy. There's a small number of owners of the mainstream press, and they will not print anything that directly contradicts the interests of these owners. This has no allegiance to any political party or ideology other than a desire for certain individuals to increase their personal power.
Various governments have allowed mergers and acquisitions among news companies until there's very little independent press. Most countries don't want to regulate press freedom too heavily (for good reason - there's a very fine line between regulating truth in journalism and forcing propaganda and it's incredibly easy for the former to slip into the latter), so we're left with the majority of the population being informed by untrustworthy sources.