Mark 5.3-4: He lived among the tombs. And no one could bind him anymore, not even with a chain, (4) for he had often been bound with shackles and chains, but he wrenched the chains apart, and he broke the shackles in pieces. No one had the strength to subdue him. (ESV)
The demon possessed man that Mark describes was a person of enormous power. It appears that he escaped confinement as a prisoner by means of his brute strength. This took place in the general area known as the Decapolis, a name meaning “Ten Cities”. It was an area where the Roman government exercised control and I can imagine that their system of chains and confinement of prisoners was very effective and strong.
But not strong enough to confine this man. He had supernatural power and as this account goes on to describe, it was supernatural strength from demonic powers. Power beyond mere human strength.
It appears that the local authorities were willing to just let him be, since he was living among the tombs. An unpleasant place where no one else would want to be. So he could have a free run of the place. He was living in an isolated manner, so why go confront him? The risk was too great, and it probably seemed most practical to just let him be.
But in addition to his power, he was a demonically insane lunatic. He was ruled by powers which dominated his behavior and drove him to the place of the tombs where no one would naturally want to be. He was not the one in control.