Why are Vi and Nocturne not played as much as they should be?

They both lack the followup to kill through the amount of CC that's everywhere. Noc can't shield everything.

At the point at which you need your team to help through some cc layers, why not play someone who can set stuff up better without needing to use your ult cooldown?

Why play a diver when you can play an assassin? A diver is just an assassin without the ability to escape AND engage. are all divers; they make the choice to either engage OR escape. and WOULD be divers, but their engage resets on a kill/assist making them assassins. (And dont say AP tristana isnt an assassin. THere was a whole meta to prove that she was).

Nocturne doesnt have enough in his kit to be best in class outside of his R. Zac has much more spammablr engage and tank factor.

Zac, Lee Sin, Rek Sai, Gragas, Elise, Kah Zix even Yi snd Trynd can all get in and out of fights in various ways. Nocturn gets stuck.

Vi was nerfed and her play rate went down when she became less dominant.

There is a strong divide in jungle tiers that commonly trends in the direction of Can you Wall Hop often and get in and out of fights or can you not?

If you look at the kits like Rammus. Why play Rammus when you can play Zac or Lee Sin. Rammus is so much less dynamic in how he can traverse the map that unless he is vastly superior in face to face conflict, which he's not, then he is outclassed. Same goes for Xin Zao.

To be viable their base #s have to be overtuned, especially for Nocturn.

Got weaker because of the jungle xp nerfs, gutting his lvl 6 rush. And honestly a very weak lategame.