While Samurott is certainly usable in OU, there are others with better stats, better movepool and better abilities out there unfortunately. Against the top defensive threats in the game Samurott misses out on certain 2HKOes and against the top offensive threats it has to use Aqua Jet which again just misses out on crucial OHKOes which means it has a really hard time at the top level. The difference isn't large at all but it is significant because it is the difference between being able to sweep and being walled or revenged every time. Sometimes the difference can be a few stat points which makes a Pokemon great or unusable in a tier.
The main difference isn't even stats though, it's just how well a Pokemon can do its job. Skarmory and Mantine being the prime example: both have identical stats but Skarmory is arguably the premier physical wall in OU while Mantine isn't used even in NU. It's because Skarmory has the tools it needs to be a wall and to support its team (such as Roost, Whirlwind, Spikes...) as well as a fantastic typing while Mantine has none of those so it's not as useful and most people find a better Pokemon that can fulfill that role, even with inferior stats.