Most mid laners have gap closers (Zed, Talon, LeBlanc, Katarina), some sort of stun/root (Veigar, Syndra, Lux, etc.) which makes Riven struggle with a 50/50 chance in Mid lane.
Top lane, Riven is one of the more mobile top laners without any resources, the others being Renekton, sort of Kled and Garen. Plus she has a shield and two forms of CC which make her capable of being retard proof in a lane where most champs are immobile.
Yasuo on the other hand has a high mobility spell that let's him dash through units allowing him to harass for free if he's not being played by a retard tryhard Yasuo main who sits at his computer in the kitchen with his fedora and cheap Chinatown Katana next to him. He also has a windwall which is a literal get out of jail for free card v. most of the mentioned champs.