Imagine your putting hardwork into mastering a champion such as azir, akali, or even zed. Then all of a sudden after thousands of hours of mastering that champion...BOOM 1st time malphite on the enemy team presses r and literally kills you with 0 counterplay at all. Imagine how that feels, it feels terrible, and ruins all the fun, when you're not having fun, you stop playing.
Oh but malphite might be picked in worlds and that would be interesting EHHHH WRONG. I would much rather watch the best players in the world play something that they can show their skills on such as stated before, azir, akali, or zed. Then have the best players in the worlds skill be wasted and instead play malphite and just press his r key off cooldown for the entire game and annihilate an enemy of his choosing. That gets boring right?
Pro play gets boring to play, pros quit. Pro play gets boring to watch, people stop watching.
If you look at all of this, bullshit like malphite being meta will literally over time kill league.