Because slither.io has VERY laggy servers, since they are hosted on a browser unlike Agar.io. Plus, slitherio is very glitchy, so people die either though a snake never even touched them.
Slither.io is too boring. Agar.io is better.
Vertix.io >>> slither.io >>> agar.io >>> snake.io
You don't slow down over time, and it's easier to defeat other players, including the bigger ones.
Like agar.io we dont have to to be bigger to win. But in slither.io we can kill big snakes with small ones.