People already watch CSI, but they also watch CSI: Miami and CSI: NY. Same thing with the 3 Law and Order shows. Face it, in TV, just like many other forms of entertainment, you have to copy what works. That's just the way they do things, and quite often it pays off.
I actually prefer CSI Miami a bit more to the other two, just because of David Caruso's badass-ness. Each CSI show has a different feel to it, as far as atmosphere.