Either Russians were sleeping or they were blinded somehow so Ukraine could sink the ship. 

That museum piece was routinely circulating the same water way in a very predictable manner because of mines without escort. It doesn’t take a genius to figure out how to ambush it especially with US intelligence behind it. quite honestly Russian’s performance in this wary really shocked a lot of ppl. In the age of drones and information warfare, they are still fighting with Soviet doctrine.