hard cc combos way better and does tons of damage and teams with a lot of it are usually filled with comps that have people like karthus who just goes to the middle of your team and then by the time your out your dead that's what merc is for. knockups usually don't last long and usually can't be aoe comboed on the entire enemy team for 7 seconds and the champions that go into these comps usually don't do as much damage. Suppresion is also op there's a reason they allow WW ult to be interrupted. Even before WW got devourer he would have been op if teams weren't on the watch to interupt his ult I remember watching a stream and he smurfed and his team never interupted WW ult when they should and that's why WW was a lot stronger in low elo's. also hard CC's are usually used offensively as stated with more champions that cc and people with aoe damage like annie/ammumu/karthus whereas knockups are used for both offense and defense so some champs have knockups that can be used to engage more effectively wheras some are purely defensive and the only champions you usually use a hard cc on from like sona are on champions like master yi or zed that kinda already get screwd if you just exhaust them.
so having a lot of hard cc like ammumu/annie/sona paired with something like fiddlesticks and karthus is a lot stronger then teams with tons of knockups the reason is CC and aoe damage stack really nicely and makes full offensive stomps easy and even though you can get merc treads it usually still hard to deal with especially if they get a lead and usually you can even have a vayne or draven player in those comps as well since they can just sit there and hit your team while your cc'ed. The only reason to use a hard cc defensively (there are exeptions) is when used on something like a fed master yi or assasains who even then already gets countered by exhaust and assasains use to complain a lot about barrier.
Sure yasuo can do tons of aoe damage while ccing your team but even then he was mainly strong when he either got fed or was combo'd with teams that had tons of knockups that could also be used offensively like hard CC and just like hard cc in those comps you used your abilities to engage and even though knockups are usually shorter then things like ammumu and sona ult yasuo was strong because it lasted quite a while and did tons of damage and that's why yasuo got nerfed by like and his base hp is like 50 less now and that's why you hardly see him. which doesn't sound like a lot but the same thing happened to jax it litterally makes all-ins and surviving and snowballing harder so now yasuo can't get as fed as easily and just gets exhausted and dies.
and then janna the only thing she does is knock back and even then there are ways around it like picking of an enemy and then it's 4v5 and since janna does no damage she can't keep you from taking turrets or taking barron though hard cc even in a 4v5 could. When fed a janna on your team can usually only be used defensively and almost impossible unless you caught out like one lone enemy to use offesensively and in teamfights almost impossible and usually ends with you dieing anyway.
and even vayne with codemn usually doesn't do well in teamfights because then even when she's fed half the champions in the game can flash cc her and then she just dies. Though vayne does excell in smaller fights or in dueling and then like janna can cancel that out but that's all she does and if one item litterally canceled everything you did that would be op. Imagine for instance if ninja tabi canceled 35% of your entire enemies damage though last whisper does that it is a strong item and there's a reason people buy it. Though there also other reasons why last whisper is so strong as well it has to do with the fact that adcs are suppose to lower health bars and buying enough armors multiplies your health by like 10 and you still usually do quite a bit of damage with things like mundo with his aoe and cleavers and even AA's to some extent and shyvana with dragon form.