'Democratic centralism' means that each of the 'communist' parties has its own, single, line on the main political issues, obligatory for all members in their public activities. The parties disagree with one another on many of these issues. If they were to merge, there would be internal ructions over control of the 'line', before they inevitably split again. Left socialist politics is like that. Even the SPGB split a few years back, and neither side there seems able to explain what that was about...