We know that this particular murder received a lot more media and police attention than the hundreds of others of murders that happen every year in New York.
I think the difference with this murder boils down to how people have reacted to it. Normally when there's an act of violence like this, people tend to think something like 'that could have been me' - being the victim. The difference in this case is that many people are thinking the same thing, but putting themselves in the position of the shooter instead of the victim.