I find that people always want to make things like personality theory into a black box: you provide a little information as input, something happens in the middle, and you expect a lot of output. Furthermore, the smaller the ratio between input and output is, the more mainstream the algorithm is.
Astrology: the time you are born -> wide variety of predictions and traits. very popular
MBTI: simple test -> wide variety of predictions and traits. not as popular as astrology
Socionics requires a lot more input than MBTI or Astrology, and thus it is not as popular. Socionics is weird in that it kind of inverts the input/output scheme. Instead of moving from less to more, it moves from more to less. In order to find out your socionics type, you need to sufficiently understand the theory and yourself. Then you can type yourself.
The difference is obvious:
MBTI and astrology promise to reveal shit you don't know about yourself by inputting a lesser amount of shit and getting a lot of shit back.
For socionics, you have to know about yourself to get a type. Thus it is more work. Furthermore, what use is a personality system if instead of giving you insights about your personality, it expects you to find the insights yourself? Then it is just useful as a label, so that you can tell more about each person by the socionics type they label themselves with.
I also notice people in the community are drawn to certainty and avoid ambiguity. There is no ambiguity to astrology for example, your birth month can not change. MBTI is a lot simpler, thus there is less ambiguity. There is a lot of ambiguity to socionics.
Even though socionics is more dynamic/ambiguous than the others, I still think that the attachment to certainty in this community is why people believe that a socionics type can not change. Honestly, I don't see any reason why it can't change. People can say "oh your true type was just hidden this whole time" but this in my mind holds the same level of validity that the type just changed. It's also a pretty liberating thought.
Oh all the people who did decades of research in developing the theory say that type is fixed from birth? Don't care .
(Yes I'm aware that a lot of socionics work is untranslated in Russian. Though I hold the lack of initiatives to translate it as a lack of interest in the West)