Because strong players gather here with 〜ので

Grammar Explanation: Objective reason with 〜ので

Cocoa inquires about the board and pieces sitting in front of Chino. Chino says it’s chess and gives a further explanation.

チノ:: 「この温泉(おんせん)は強者(きょうしゃ)が集(あつ)まるのでおじいちゃんは白熱(はくねつ)していました」; "Grandpa would get excited because strong players gather at this hot spring spa."