The main reason is mostly because original "compressed and expressed as text" format would be a problem in a case of svn conflicts.
Therefore the new format is more text-like and only when it is required to compile topic group into the project, which is in fact a minority of topics, you can mark such topic group as "includeable" and it will produce additional compressed files.
(From the C++ source code, nothing changes, it is still about *.tpp/all.i include).
Opening topics for wide contributions is therefore a bit closer again....