" ... We seem to be faced with a pendulum movement in history from absolutism to democracy, from democracy back to absolute dictatorship.
"The amount of individual freedom which a people may conquer and keep, depends on the degree of its political maturity. The aforementioned pendulum motion seems to indicate that the political maturing of the masses does not follow a continuous rising curve, as does the growing up of an individual, but that it is governed by more complicated laws.
"The maturity of the masses lies in the capacity to recognize their own interests. ... Woe to the fool and the aesthete who only ask how and not why. But woe also unto the opposition in a period of relative immaturity of the masses, such as this. ... In periods of mental immaturity, only demagogues invoke the 'higher judgement of the people'." [author's italics]
If I understand your question of "why" correctly, as in his intended objective as opposed to his motive, Rubashov wrote "Theory of Relative Maturity" to explain, stated in brief, the tendency of people to be oppressed and the corresponding tendency of other people to oppress. Rubashov analyzed history and observed patterns, which he perceived as connected with recurring historical problems. He discerned a way to explain these through a theorized hypothesis of psychological and sociological behavior. Thus he wrote his theory to propound the theorized explanationof the problem for other theoreticians and, eventually, for the world. Rubashov analyzed that their era had past the peek of the pendulum swing toward absolutism and was continuing the descent into a settled immaturity from which a new pendulum swing to the opposing arc point of democracy might emerge at some vastly distant time in the future. Therefore, political and sociological freedom is relative to maturity, which itself is relative to the position of those pendulum swing impulses moving between absolutism and democracy:
... philosophical incendiarism had given place to a period of wholesome sterility. Revolutionary theory had frozen to a dogmatic cult, with a simplified, easily graspable catechism ....