Man pages are like scripts in that they need to be placed into a special directory at the top of the install hierarchy (they go into prefix/man rather than prefix/bin). This is easy to do in distutils or the original version of setuptools because the directory path for data_files was assumed to be relative to the prefix. However, in distribute the directory path for data files is the egg itself. This cannot be used for man pages as they must be placed in a directory named 'man' that is immediately adjacent to the scripts 'bin' directory in order for them to be found by the man command. Trying to work around this issue is very difficult and error prone.
This represents a loss of functionality (distribute cannot handle installs that include man pages) and a backward compatibility issue (install scripts that worked before distribute is installed fail afterwards). This is a serious issue because all executables installations are expected to have man pages.