See, for example, AT&T's eleven-years-and-counting ordeal to roll out U-Verse across San Francisco.
1) they can deal independently with multiple, local bureaucracies, each of which is easier to deal with than a consolidated large city's.
2) there's a ton of high tech / biotech here already, and demand for fiber.
3) The population is highly educated and there are several very good universities (UNC, Duke, NCSU) and smaller schools (Shaw, Peace, Meredith, NCCU, ...) in the area.
4) With the exception of older parts of Raleigh & Durham, all of the cities are pretty well planned and have layouts that make running fiber pretty straightforward, and all of the municipalities agreed to accommodate all of Google's requirements.
Coincidentally, we had 0 fiber ISPs a year ago and now we have 3 (plus TWC is updating all their "extreme" customers to 300mbps). AT&T has been laying fiber like crazy and Centurylink is here, too.
That's true of New York, but I'll also note that NYC has reasonably good Internet speeds in much of the city right now. For example, RCN sells 330 Mbs / down and ~25 Mbs /up for $~60/month right now: http://rcn.com/330. Verizon FiOS is more expensive and probably worse (unless you need high upload speeds for some reason), but it is widely available.
Google Fiber is probably not targeting places that are "pretty good, but could be better;" they're probably targeting places that are a combination of "really bad," along with reasonable regulatory climates.
That being said, there are neighborhoods that don't have anything close to this level of service in NYC, but I doubt they would be targeted by Google Fiber in the beginning.
Not to mention the fiasco that FiOS is going through right now is probably a big warning to stay far far away.