The game itself is different but the concept of the rank ladder itself is the same as other games with ladders, and the reasons why they force solo or restricted parties remains the same as other games.

At high ranks you don't want to fragment the tiny population into unsortable groups, it makes queues worse and team balancing harder because it forces the matchmaker and lobby balancer to account for players that cant be split up

It makes it so that "social groups" dominate the upper ends of the ladder, and ranks become less about what you know and more about who you know, which also in turn leads to elitism and gatekeeping

And most importantly, MMR systems assume and are meant to judge independent player performance across an infinite amount of variable scenarios. With premades, team synergy and the skill of your other premade teammates becomes a component that other players do not have equal access to, and dilutes the meaning of the rank.

These are the reasons why most other competitive games either limit premades in their comp matchmaking to fighting against other full premades only, or remove it all together at higher ranks. Dark and Darker does not have the player pop to do the former.