John Gill Commentary 2 Chronicles 20:33

"Howbeit the high places were not taken away; neither as yet had the people set their hearts unto the God of their fathers." — 2 Chronicles 20:33 (ASV)

Howbeit, the high places were not taken away
The high places, where idols were worshipped, were taken away, (2 Chronicles 17:6) , but not those where sacrifices were offered to the true God:

for as yet the people had not prepared their hearts unto the God of
their fathers ;
to seek and serve him wholly, according to his will, to offer sacrifices to him only at Jerusalem, as the law required, (Deuteronomy 12:5–7) , they could not as yet be prevailed upon to relinquish the high places, built and made use of before the temple was; partly because they were nearer to some of them than that, and partly out of veneration for them, as being of a long time used.