As someone who loved RDR2, played a ton of GTAO with friends, and only played some of RDO before putting it down, my pet hypothesis is that the friction level of in-game travel is too high for RDO to be a good multiplayer game.
In GTAO, you spend most of the time using a vehicle to travel between places, which is easy because you just hold down the “accelerate” button and point your vehicle in the direction of your destination, and funny stuff happens along the way. If your car gets wrecked, no big deal, just grab another one. If the cops get involved, it’s often trivial to escape them, or you can just call Lester if you’ve unlocked that functionality.
In RDO, you also spend most of the time travelling between places, but it’s way more annoying because you have to tap-tap-tap-tap-tap the button to keep your horse moving, unless you want to move at the default pace. The “funny stuff” in GTAO doesn’t translate to RDO. Crashes between horses/carts/wagons can be disastrous in terms of continuing to travel to your destination: horses may die or run off, and there may not be another horse/cart nearby to steal. (And if you do manage to steal a horse/cart, I think RDO has an imposed character morality system that GTAO obviously does not.)
The design choices that totally work for an often slow-paced single-player game where you’re encouraged to take your time are a real hindrance when you have two hours after work and want to mess around with your friends in an online game.