I think it is a hard transition to the NFL in lots of ways, and that most players are well served by having a chance to sit a year to get their feet better. I think Zach was thrown into a rough situation with a terrible media and that he hardly took the transition in stride.
I also think that the Jets, who had been truly terrible, started to improve at a quicker-than-expected rate. Zach was, for all of his struggles, a part of that improvement; he was able to not lose games, provide some sparks, and do well in flashes. Unfortunately, the Jets fans, driven by the misery of living in New York and by the media who love being in a furor, began expecting more and more. Then, when their running back got injured, fans looked for a scapegoat and the Jets gave them Zach.