In order to keep people into subservience, every ruling elite needs an ideology whose main purpose is either to promote this state of subservience as something good, or to promote things that have the ultimate result of keeping or worsening this state; in both cases: an ideologies mainly based on falsehood. On the contrary, in order for people to emancipate themselves, they need an ideology that educates people on what's actually good and what's actually bad, and therefore is mainly based on truths.
However, any ideology that is almost exclusively made of falsehoods is too easy to unmask, while any ideology that is almost exclusively made of truths is too easy to recognize; as long as the philosophical spectrum keeps being so clearly divided along those lines, sooner or later the entire population will eventually flee from the regressive ideology and flock to the liberating one, no matter the amount of propaganda employed to defend the first and/or attack the latter.
For this reason, the modern political ideologies are systematically manufactured in order to always include a specific amount of truths among the lies: each ideology is designed to speak the truth on a specific set of subjects and lie about the others, while the opposing one is designed to do the opposite and tell the truth on the latter while lying about the former. In this way is much more difficult for people to recognize which ideology is in the right one, mainly because *none* of them is: regardless of which side is chosen, the truth contained in each one is neutralized by the rest of falsehoods (logically speaking, true and false = false) and will therefore pose no threat for the establishment.
People will usually choose their ideology on the basis of which subject resonates the most with their own personal psychology or life experiences: truths that happen to confirm a person's own personal beliefs will be recognized much more clearly than others; he will latch much more strongly to that truth, and simultaneously also to the whole ideology associated with it; all the included lies will instead be ignored since they're about subjects who are not perceived as strongly as the other one. Instead, he will promptly recognize lies about his "favourite" issues made by the other ideology and start crusading against it, including against all its truths about the "unimportant" issues, who are left unrecognized for the same reason.
All with the additional effect of dividing people and keeping them fighting among themselves rather than against the elites.