Maining Riven is WAY. WAY. WAY. to rewarding to the players for one. And maining her takes rewards to a new level to the point where enemies can barely do anything about it. I myself mained riven a good while and got her to my third most mastery points and almost hit gold because of it, A friend who was very bad at the game (no offense) started maining Riven and actually went on a HUGE WINSTREAK, LIKE HOLY FUCK I'M TALKING A GREEN MATCH HISTORY PAGE ALL POSITVE SCORE GAMES, VICTORY VICTORY VICTORY AND ONLY 1 LOST GAME OUT OF IT ALL. Went from bronze to platinum like it was nothing, And then I myself did the exact same thing, And made sure at every tower to kill the minions that came before attacking again.
Rivens hardcore strength comes from her not having any resource bar, She can farm with abilities and clear waves like it's nothing in no time, Meaning you can't hold her back from power farming, I also began invading the enemy jungle as much as possible whenever I was pushed up. and with Electrocute I was near unbeatable in 1v1's Even against my main counters such as Garen and renekton.
Riven is also a late game champion meaning even after she stomps you in lane you have no real chance of fighting back, She has 4 dashes, a knock up a stun a shield and an execute ultimate that gives free AD. Abused in high and low elo, Easy to climb with and theres a reason why So many Riven mains are part of the challenger rank.
Edit: Theres currently around 5 Riven mains in top 100 Challenger.