Some keyboard shortcuts for VCS are not mapped between different VCSes.

When I assign keyboard shortcut for 'Show History' in Mercurial the same shortcut is being assigned automatically for the same command in other VCSes. However if I assign keyboard shortcut for 'Tag Repository...' in Mercurial this shortcut is NOT being assigned for 'Create Tag...' in CVS nor for 'Tag Files...' in Git.
What is the reason 'Show History' is being mapped between different VCSes whereas other commands are not?