Why did God say David was not to build the temple?

Answer: He had shed much blood

God told David he had shed much blood and waged great wars, so his son Solomon, a man of rest, would build the house.

“but this word of the LORD came to me: ‘You have shed much blood and waged great wars. You are not to build a house for My Name because you have shed so much blood on the ground before Me.”— 1 Chronicles 22:8 (BSB)