Sforno on Exodus
מדוע אתה יושב לבדך?, why are only you dealing with public affairs?
Rashbam on Exodus
מדוע אתה יושב לבדך?, and as a result of your being the sole judge the entire people have to stand in line before you from morning to evening.
Mekhilta d'Rabbi Yishmael
(Exodus 18:14) "And Moses' father-in-law saw": What did he see? He saw him sitting like a king sitting on his throne and all paying attendance upon him, whereupon he said to him: "What is this that you are doing to the people? Why are you sitting alone?"