Everyone knows that New York City is the most important city in the world. The laws made concerning the city by New York state are just for that, the down state. Now, why should the laws of NYC be enforced on the people of upstate New York, a mostly rural area with a completely different way of life then the city.
The Upstate has a very different way of life then the down state, with people who have a different mentality then their downstate counterparts. The upstate would be better off enforcing laws for it's own people; not laws made for NYC that affect the entire state of New York.
Splitting the state would stop issues of down state laws effecting an area where a majority of the population disagrees with the law, an example of this is the SAFE act, which is not supported upstate.