The teams he's played on have immediately gotten better after he's left as well. Look at how the Knicks are doing, how well the Blazers started playing. There's two examples right there of teams who have gotten better after he's left the team, and if it's because of the trades made, then with him being the one being traded, obviously that doesn't help him either. If it had just been a bad team that was bad before and bad after, I could see that, but this hasn't been the case for the most part, although I'll be willing to concede the Knicks for the same reason that I'm not praising him yet for the improvement of the Clippers. There's good reason that Randolph has a bad reputation.
The Knicks were on a good season before he left the Knicks. Their strong play is because of D'Antoni. They only didn't want him because of his contract length.
The Blazers got better when he left because it gave LaMarcus Aldridge a chance to start, so there was really no downgrade at all. Also the team was still the youngest team in the league at the time, so they had gotten a year better. Randolph leaving had virtually nothing to do with the team's improved record.
Randolph's not Stephon Marbury. He doesn't make a team bad. He's a guy who needs a lot of shots but isn't a leader. Those two things don't usually mix well in the NBA. GMs and coaches like the guy who takes the most shots to also lead the team. Zach just isn't a superstar. That's all there is to it.