Because I like making a point:
•AOE cc for an ult.
[Sejuani]
Range: 1175
Max Dmg: 350 + .80AP magic
Max CC Duration: 1.75 seconds
Target type: AoE Colliding Skillshot
Notes: Can initiate at range, can stun multiple targets, Sejuani can remain at safe distance (can be a disengage), but has to hit target to stun, can miss (slows only it targets are in range of AoE)
[Vi]
Range: 800
Max Dmg: 450 + 1.4 Bonus AD physical
Max CC Duration: 1.25 seconds
Target: targeted (point & click) dash
Notes: Can initiate at range, can knock aside multiple targets but generally only used for 1 target, Vi moves herself into range (generally not a disengage), cannot "miss" (unless shielded)
[Nautilus]
Range: 825
Max dmg: 450 + .80AP magic
Max CC Duration: 2 seconds (0.5 for secondary targets)
Target type: Targeted AoE
Notes: Can initiate at range, can knock up multiple targets, can give large zone control (drop Ult on back line, watch everyone scatter), can be a disengage, cannot miss (unless shielded)
[Nunu]
Range: 650 (radius)
Max Dmg: 1,125 + 2.25 AP Magic
Max CC Duration: Channel time of 3 seconds
Target type: point-blank AoE channel
Notes: can be interrupted for less damage, can be avoided by moving out of range, Nunu has to already be within range/danger zone (not a disengage), Channel prevents Nunu from doing anything for 3 seconds.
I picked the first couple names in your list. Even though each one has a "CC Ult" they are each radically different. Sejuani's Ultimate has the most range and is an AoE Stun. Why? Sejuani's other abilities are very short or melee range, so this is her only range engage. It also lets her use it and stay out of the fight if she needs to, or if she's out of position. Compare that with Vi - Vi's ult puts her directly next to an enemy. Now usually this is exactly where Vi wants to be - but it means that if Vi is low and trying to get away, she can't use her ult in the same way that Sejuani can to protect herself or allies. Now Naut, on the other hand, can still use his to engage or disengage, but it also has a lower range than Sejuani, and usually gives the opponent plenty of time to react at range (long wind up animation, his ult has a lot of travel time if the enemy is running away). Why? Well, Nautilus (unlike Sejuani) has another ranged engage (via his hook) so its okay to give him an ult that isn't quite as quick or powerful as Sejuani, because Naut already has so much lock down potential. Then you have Nunu - Nunu's is the least reliable of all them - it takes a full 3 seconds to reach max damage, it can be interrupted very easily or they can just walk/blink/dash/Zed right out. Why? Because look at that damage. 1,125 base damage. Ouch. Nunu's ultimate is all about "Focus Me Or Die", forcing the opponents to either flee in terror or focus their attention on stopping/killing him, which is exactly what a Tank is meant to do. Notice that with Vi/Naut, the focus is more about locking down a single enemy target so that your team can kill them, while Nunu is about forcing the enemy team to move their focus off his allies.
I could go on for your entire list, but this should make my point clear enough. Each one has its own niche, its own role to play. And you'll start to see after a while why you'd pick one Champion over another. Got a lot of AoE lockdown ults like Amumu, Orianna or Kennen? Grab Sejuani. Enemy have a strong hypercarry like Vayne that you need to be able to lock down? Grab Naut or Vi. Enemy lack hard CC? Grab Nunu and make them pay.
This same thing goes for other roles. Look at ADCs - a lot of them have some form of 1) damage amplification, usually in the form of AS boosts (Graves, Jinx, Varus) or 2) critical-strike synergy (i.e., they auto attack), or 3) if they don't crit, they have some form of "Alternate Damage" such as Vayne's Silver Bolts or Kallista's Rend, and 4) some form of mobility/utility to reposition or keep themselves alive (Tumbles, dashes, blinks, spell shields, etc) - but each of them are vastly unique.
Melee ADCs are even more "color by numbers" than probably any other class. Riven, Tryndamere, Yi, Yasuo. They all have:
- gap closers (Riven's multiple Qs, Yasuo's Dash, Tryn's Spin, Yi's Alpha Strike)
- self-defense or damage mitigation (Riven's shields, Yasuo's shield/wall, Tryn's "I Can't Die" Ult, Yi's Meditate)
- Damage amp (Riven's passive and mad AD scaling, Yasuo/Tryn crit passives, Yi's double-strike passive & Bonus AD boost & true damage)
- way of sticking to the enemy to stay in melee range (Riven's stun, general mobility, tryn's AoE slow, Yasuo's repeated dashes/wall to block CC, Yi's "Cannot Be Slowed" Ultimate)
But each of them brings something different.