He didn't turn away from the sins of Jeroboam. Zechariah ruled Israel for only six months before Shallum murdered him and took the throne. This ended the dynasty of Jehu after four generations of his descendants, fulfilling the prophecy in 2 Kings 10:30. Floyd N. Jones suggests that Zechariah may have reigned 12 years. He argues that 2 Kings 15:8 may indicate that Zechariah did not begin reigning in the 38th year of Azariah, but that in his final year he reigned six months into Azariah's 38th year.
Floyd N. Jones
Zechariah