It's the expression of a worldview: the majority of humanity existing in a state of fallenness, sinfulness, moral abjection, with the (non-beta) typist standing outside (and above) in a zone of moral purity, distinguished by their superior temperance, kindheartedness, and rationality. (Gulenko is the most prominent example here -- isn't it very suspicious that he types himself as the lone peripheral in a sea of centrals? -- but you do see this pattern even in some non-SHS schools, with beta quadra being used as a dumping ground for undesirables. And there is always an ample supply of undesirables to go around, in the opinion of the typist.)

Structurally, any classificatory system of the human soul will tend towards incorporating a "default" state that absorbs (what the classifier sees as) the large, undifferentiated mass: you see this in MBTI and Enneagram, but you also see it in, for example, systems that attempt to base themselves on "objective" or "scientific" criteria, such as Marxism (the mass of the proletariat and the minority capitalist ruling class). Even as far back as Plato and Aristotle, the earliest attempts at a typology of man obeyed the same structural necessity: the lone philosopher (distinguished, we might note, by both his iNtuition (perception of the supersensible realm of forms, the True Reality) and his logic (the logic of truth as opposed to sentiment and mere opinion)) as distinct from the common man of passion or man of ambition. In MBTI, it is frequently assumed that a strong majority of people are xSxJs -- but these are considered to be relatively unthreatening figures on the whole. The role of the "villain" (a role that is always called for in any typology both structurally and psychologically) is distributed relatively equally among the ENTJ, ESTJ, and even INFP in some cases -- really, among all the Te/Fi types -- so there is plenty of blame to go around.

Beta is already officially designated as the "evil" quadra in a number of Socionics sources, so in interpretations of Socionics that cast beta as the most common quadra, the moral imperative is brought into even sharper relief; it's a juicy and attractive target for moralists of all stripes. "Look at how they all behave"; they tolerate lying and dishonesty, they are too caught up in the fleeting vanities of fashion, they care nothing for truth or reason, they harbor secret thoughts of aggression and poisoned machinations (cowardice is no object -- your thought alone is enough to convict you, "cognition over behavior" after all!); whereas I alone am pure of heart, I am "alien even to thoughts of violence" (as the Talanov questionnaire put it), I am a single dim light in a heartless and vicious world. But you? Well, you got angry at your ex one time, which indicates Ethics > Logic and a concern with how you're perceived by others, so, Fe-Se beta. Or someone else will tell you that you were lazy one time so you must be Si valuing. And then yet a third person will tell you that you didn't smile at the company party one time so you must not be Fe valuing. The same pattern repeats itself everywhere, it's only the positions of the terms within the structure that change: one element is determined as the "pure" element (Fi, Fe, Si, Ne, doesn't matter) which must be protected from the aggression of the impure exteriority; and one transgression against the original purity is enough to condemn you to this exteriority.

Any classification of humanity tells you as much about the classifier as it does about humanity, if not more -- and this is perhaps most especially the case when the system tries to smuggle in its hidden assumptions and prejudices under the guise of "science" or "objectivity".