They were shopping him because he was increasingly giving management the impression that he did not want to be in NY and go through this rebuild with the team. They were shopping him because after this next season he would have the option to become a free agent and the Knicks would risk losing him for absolutely nothing.
Now as many have already said, you can blame the FO for not being able to heal the relationship and convince him to stay onboard. That's fair.
At this point there's enough information surrounding the circumstances to not waste time with "hot takes" like this and pretend there weren't adequate reasons for the Knicks to explore possible KP trade opportunities.
I freely admit this trade could turn out disastrously for the Knicks. But the plain truth is that this is not a trade that is getting widely panned by basketball minds across the league. Other than mostly fans who pay no attention to the knicks other than to throw in a random ROFLKnicks every time the team is brought up, the widespread consensus is the Knicks did the best they could with an unfortunate situation.