Each tag set community has different focuses, practices, and values based on their use cases. For example:
DITA and DocBook are typically concerned with content creators creating content: authoring (and reuse) by people.
JATS and BITS were designed as conversion targets; they typically assume that content already exists (probably created in Microsoft Word or legacy in PDF) and needs to be marked up for interchange or archival purposes. (There is an authoring version of JATS, but it is very rarely used.)
TEI typically deals with previously-created or historical content, marked up by subject matter specialists who are content analyzers and annotators.
Such differences in the who and why lead to significant changes in the mindset of the designers and developers, in the structure of the tag sets, in the nature of use, in the documentation, and in the tools created and shared.