Compared to other parts of your body, the tissue inside a tooth makes for a special case.
When it becomes inflamed (like in response to advancing decay or other type of insult), its blood and lymphatic vessels that are needed to transport white blood cells and nutrients to it for defense and repair become restricted (compressed by the accompanying swelling that occurs within the tooth).
In those cases where the pulp tissue's attempt at recovery has failed and it has ultimately died off, conditions are even worse. Now there are no longer any vessels inside the tooth to efficiently transport the white blood cells needed to combat the invading bacteria.
That means a necrotic (dead) tooth can make a relatively cozy home for bacteria. It's a place where they can live, yet not be effectively challenged by your body's defense mechanisms.