Why does Fiora have high CD's and mana while Riven doesn't.

The new Fiora's kit is great. I see nothing wrong with it's design. But it feels like I'm playing a yasou or Riven type champion, except I'm given a bigger handicap with really long cooldowns and restricted by mana. I'm not against this in theory, but I don't see the logic why Fiora's kit gets these limitations but not champs like Riven, Yas, Renekton, etc. It's not like her abilities are stronger than theirs.

I can see why Darius has cd and mana limitations because he hits like a truck, but Fiora really does not.