When you have a massive advantage against the enemy top laner, and even if you fall behind in the side lane, you should still be able to win the 1v1.
For example, against squishies like Jayce or Gnar, you can win the lane, and even if your items are worse than the enemy's, you should still be able to win if you just hard-engage.
Against champions like
who guarantee a low floor even if you are winning the lane, if you fall behind in items, they just run you down and beat you up. They are hard to break through, have good CC, are tanky, so you can't even kite 2v1 ganks easily, and you die quickly whenever they come to gank.
Enemy mid laner has few or no guaranteed CC abilities.
If they don't have guaranteed CC, it's easier to kite ganks in the side lane and beat them in a 1v1.
The moment they drop Lego or Tibbers on the ground like
, you're dead. You can't do anything in team fights either.