Adding to the oversimplified (but 100% correct) answer that Jarjar gave, Samurott has a low base stat total, meaning it's just not good enough in the higher tiers which have Legendary Pokemon and other Pokemon with great stats. Another thing is thatall its stats are diversely spread, which is a good thing in-game but in the meta it conversely means it's not excellent in any one particular stat. Such weakness is punished in the fast paced meta game of the higher tiers, where Pokemon exist with stellar stats in just two or so stats and play that specific role on the team, be it just attacking or just defending.
Very few Pokemon can do both well enough to be OU as a tank, and those are psuedo or Legendary Pokemon (some exceptions always present). Samurott simply doesn't have the stats or the moves to compete with that sort of crowd.
Lastly, in case you're wondering why the other two are ranked higher, the answer lies lies in their excellent hidden abilities which benefit their main STAB ( Contrary Leaf Storm and Reckless Flare Blitz), which boost their performance in their respective tiers, something Samurott got the short end of the stick of.