There are often legal barriers to transferring prisoners out of a state, not least that it imposes a significant burden on their families who may be unable to visit them (and who have rights of their own, regardless of how one feels about the prisoner).

I should have mentioned that states like NY als have more extensive rehabilitation programs (which cost money); on the upside, as far as I recall NY has one of the lower rates of incarceration and recidivism (but I might be wrong, don't feel like delving into the stats right now).