Because it is a blockchain game where you can play-to-earn. It also have the highest users according Dappradar (https://dappradar.com/rankings/category/games ), beating axie infinity users. The reason why users number is so important is because the game itself have alot of room to grow, and able to compete with many players. And the gameplay in Splinterlands battles are fast, with each match taking only 2-3 minutes so able to farm faster. Despite being auto-battle game, this game is so fun and addictiing. Especially if you are a fan of strategy game.