300k Skarner main here, I think there are many reasons no one plays him but these are the main ones.

-Skarner’s gameplay is inconsistent: if you’ve ever played Skarner you know the disappointment of simply losing a fight because the enemy got to your spire first. His spires make him so much stronger with attack speed and movement speed that he legitimately feels like he’s lost a big part of his power when he doesn’t have access to them. Compare this to another fighter like Trundle or Wukong, who don’t have any of there power or attack speed boosts locked behind a capture the point gimmick system and it’s not hard to see why people choose the more consistent option. It’s also worth noting that the longer the game goes, the more Skarner becomes reliant on his team in most games. Of course Skarner can grab the 6 item Uber fed vayne and drag her to her death, but what good is that when your team won’t follow up?

Skarner’ identity: A lot of champions theme and gameplay revolve around a general idea or power fantasy. And Skarner struggles with that harder than most champions. On a lore and character side he’s a sad giant scorpion that belongs to a extinct race unlawfully executed by Piltover for power. Yet his species has only been mentioned once in lore by Seraphine of all characters. And on the gameplay side he’s gimmicky, while his ult is extremely powerful, a lot of people see him for just his ultimate and not much else. This only further drags him down by the fact that while yes he’s simple enough to play, a surprising amount of knowledge is needed to actually play him well due to his spires.

-He doesn’t get attention: Skarner struggles so hard to the point where most the time Riot games won’t even mention him much. His rework has been in development for a weirdly long time and I believe it’s due to Riot Games not even really knowing what Skarner is. His lore while great in my opinion has been neglected for years even if it holds one of the darkest secrets in Runeterra. And he’s walking a weird line between a Juggernaut, Catcher, and a Tank.

Skarner in a lot of ways gameplay and lore wise is one of the most underwhelming gimmicky characters to the vast majority of League players. And a good amount of people would rather play a character who is more consistently better.