Very easy to explain this one.
He destroys low elo since people don't understand farming, pretty much. They just want to hit you over and over, which is fine. All you have to do is farm loads tanking harass, use potions to keep heath up, and when you have damage you can start shoving your lane to their tower. If their jungler comes, 2v1 them. Your lane opponent typically has terrible farm and items. If you run teleport, sometimes you can make them miss EXP on minions too.
He doesn't destroy higher elo since everyone harasses him properly whilst maintaining high farm, the jungler will come very early on and kill him a few times making him useless until extremely late into the game, and he gets kited over and over. Wither only affects one target. Since he needs to farm constantly, the enemies sometimes teamfight as 5 a lot earlier, making it a 4v5. At that point his damage is terrible. Nasus also has no proper escape, making him a pretty poor pick when kills start having a lot more meaning in the higher divisions.
Plus, there's now champions such as Kalista who completely counter his wither with those dashes, making him incredibly easy to kite.
The real question is - why is his legendary skin still completely broken? Still uses the crit voice for normal hits, Q still looks stupid, still can spam his taunts.
