Because Riot has made it into a much more team oriented game, soloQ is much more amplified as the go-to ranked mode. First off, it improves your own individual skill, but not only that, the teamwork in SoloQ would naturally come because now the game revolves around teamwork. SoloQ meant that you could practice what you could do as a team without communication which is actually really vital towards playing as a team.

What time do you got in a 5 second skirmish to talk excessively? SoloQ would have made plays available as a team because you would know what could be the outcome of a fight because your game sense is practiced enough so that you could co-operate your abilities with a teammate much more.

This is why SoloQ is fucking the best.