Why did the man who knelt before Jesus in Mark 10 go away sad?

Answer: He had great wealth

Told to sell everything and give to the poor, the man went away in sorrow, because he had great wealth.

“Jesus looked at him, loved him, and said to him, “There is one thing you lack: Go, sell everything you own and give to the poor, and you will have treasure in heaven. Then come, follow Me.” But the man was saddened by these words and went away in sorrow, because he had great wealth.”— Mark 10:21-22 (BSB)