It's just about helping everyone in the game have the most fun. Soft CC is less satisfying to give and often more frustrating to receive, but hard CC gives a more noticeable effect and more appreciable results.
Riot came up with a bunch of different kinds of CC, but found that some of them just aren't fun to play against. With the exception of Soraka, every silence is extremely old, and they have been gradually removing it from the game. Blinding is really frustrating to play against and hard to notice, so they took it away from quinn and haven't put it on any new champions.
Stuns and roots feel more visceral and satisfying on both sides. Getting stunned sucks, but getting blinded can feel even worse because it is so hard to notice. Soft CC like slows, silences, and blinds also tend to last longer to make up for their lack of potency. Getting silenced for 3 seconds can drag a fight out a lot longer, and even be more frustrating than a 1 second stun, but it doesn't feel as satisfying for the one doing the silencing.
Grounding is a new form of CC, during Riot's wave of counters to mobility creep (soraka E, cassio w, poppy w, taliyah e, etc.) so it isn't surprising it hasn't made it onto a new champion yet, but I hope it will.