The main reason for this difficulty that lies in the regeneration process is due to the anatomical structure of the root canals.
Often the canals are calcified or hunched, which makes it impossible to access them. The root system of a tooth can be compared to the one of a tree which lives on its head.
Therefore, it is normally possible to clean and fill the main root canals. But the side canals often remain disregarded and are the ideal breeding places for bacteria.