Want to know why Riot cancelled achievements? Look no further than Black Market Brawlers.
This mode + event proves why achievements cannot work in League of Legends.
Anyone who has played the game mode more than a few times has already probably suffered through prolonged games where at least one person refuses to group in a won game because, "they need more Krakens for the event".
This pretty much justifies all the fears that adding any kind of personal achievement besides "win the game" would overall decrease the player experience. Luckily it's only for a limited time event meant for fun. If these kind of personal achievements went live to all match making modes, it would really suck.