My assumption comes from educated guessing.You know what they say about assuming don't ya?
I get bored of open world games like that, I get bored of games that rely on repetitive
I get bored playing GTA games, I get bored playing Saint Row games, I get bored playing Batman Arkham *insert something here* games. I pretty much get bored playing any type of game like that, and from what I saw, RDR games are of the same type as those other game series.
Not to mention that I play Mouse and keyboard, and those games usually play much better with a controller. But for example, when I tried Saints Row 4, I couldn't remap the keys on the controller and it was awful. I'm definitely not a controller type of player anymore.
The only thing RDR has over all the other games I already mentioned, is that the Wild West is way more appealing to me than modern/sci-fi/super hero's settings.