Why was King Uzziah struck with leprosy?

Answer: He burned incense in the temple

Grown proud, Uzziah entered the temple to burn incense, which only Aaron's descendants were consecrated to do.

“But when Uzziah became powerful, his arrogance led to his own destruction. He was unfaithful to the LORD his God, for he entered the temple of the LORD to burn incense on the altar of incense.”— 2 Chronicles 26:16 (BSB)