There is million reasons I think duos should newer be in same category with solo q...will make just a couple here:
1.They newer admit mistakes of each other which makes them aggressive towards other players.For example i am top and i am duo with mid.. my premade goes 0 8 in lane and wee both blame jungler ,because i will newer admit my teammate is bad player
2.They are lower mmr because of riot policy that duos must be lower elo...Which make them more then often feed and make serious mistakes
3.Only time duos do something is when they copy some shitty strategy like lulu top master jungle ,and it is more then unnecesary to explain why that suck..It make them boost each other to elo they dont deserve just by abusing strategy.And it make game stupidly unfair for full enemy team,and horrible game experience for all except duo.Or when some smurf boost his premade which is again unfair to everyone except duo
that is just a 3 main reasons that are more then enough for riot to consider change on this aspect of game...But since I know how idiotic riot company is I dont expect anything reasonable from them...