I just posted a general explanation of its prevalence in only Japan. But here's another perspective from a Japanese psychologist who's been working with Hikikomori patients for 15 years+:
According to my clinical experiences, hikikomori has a history of attachment trauma with their mother (all biological mothers due to the absence of divorce in the families). When rejected emotionally by a mother, a young child hides his original self for protection, and creates a false self to adapt to the dysfunctional mother. The child grows to an adult who neither trusts nor shows his true identity to people. When the conflicts between pretending and hiding the true identify become unbearable, he would retreat into extreme degrees of isolation and confinement.
This phenomenon is observed commonly across my hikikomori clients. Hikikomori begins from dysfunctional mother-child relationships, which are influenced by the society that values group harmony and compliance over individual freedom. The syndrome began surfacing when Japan achieved economic success while the mothers with traditional values were unprepared to satisfy the children’s needs for love, individual freedom, and self-actualization. Hence, many hikikomoris find no place in Japan, feeling captive in the affluent but culturally oppressive society.