Sentret can be obtained extremely early, and it doesn't take very many levels for it to evolve. You have to wait longer to catch a Krabby, meaning you would have to use a different Pokemon before you got it, and you would probably have a Furret already before you got a Krabby. Furret is also able to be of some use in a battle, while Krabby wouldn't perform too well.
Also, the Furret meme is a thing. People may use Furret for the "haha funny joke" aspect it brings to the team, which Krabby doesn't have. Furret is also cuter, which can be a factor in a younger player's decision.
Hope I helped!