Because it is pointless. It was made to promote fun with friends in ranked. Being able to climb together rather than alone. It is pointless and impossible.
Imagine this scenario. 4 friends start playing Leauge, they decide to go for ranked and group up as 4. After climbing to gold 5 they realize one of them can't compete at that level of play and are left with a tough decision, either kick that guy or not care about their rank and keep playing. They kick the guy because they actually do care about their rank. The 3 remaining players start climbing again up to gold 1, where they realize one of them can't compete at that level, so they kick another one. The duo now proceeds to climb once again. At platinum 1 one of them notice that the other player can't compete at that level so he decides to take the rest of the journey by himself and gets to diamond 2.
It would be a perfect system if everyone was evenly skilled, but we're not, we're all different. Some go on to become high elo players some stay silver or drop to bronze. It might promote friendship and fun games with friends for a while, but at some point you have to decide if you want to stick with your friends and have fun or if you want to focus on your rank. Those who choose friends, don't care about their rank too much and could just as easy play normals. Those who care more about their rank don't need friends to play with because the most important part is still their rank.
Also, it becomes increasingly hard to find a large group of friends to play with the higher you climb.
I think having dynamic queue available for bronze, silver and gold would be fine, most players hang out there, finding friends is easy and a lot of them don't care much about their rank. In platinum+ though, most players care about their rank, most of us don't need friends to play with.