He did have good months at the Whote Sox as well, and his stats at the Brewers are a small sample. If he has turned it round full time then good for him and I hope he goes on to have a good career, it still doesn't change the fact that his time was up at the White Sox.
As for why he was bad for the White Sox the whole atmosphere around the club has been poor, and he is not alone as a young player coming into the team who has underperformed and a lot of expectation was heaped on him, and his development was not ideal. Some players do play better without the weight of expectation on them, at the Brewers he can just be one of the players rather than a first round pick who was expected to form a cornerstone of a rebuild.