Albert Barnes Commentary 2 Chronicles 21:20

"Thirty and two years old was he when he began to reign, and he reigned in Jerusalem eight years: and he departed without being desired; and they buried him in the city of David, but not in the sepulchres of the kings." — 2 Chronicles 21:20 (ASV)

Not in the tombs of the kings - Compare the similar treatment of Joash (2 Chronicles 24:25) and Ahaz (2 Chronicles 28:27).