It may be due to the ubiquitous stairs and subways, but this device would be utterly impractical in most situations here, and it looks silly to boot.
The company I work for had a contest for our customers where we gave one away. I got to ride it around at work a bunch (I was the only one that could successfully ride it without risking a broken bone). The guy we gave it to drove it into his pool. Oops.
The other one belonged to a lady that used to live near me in Scottsdale, AZ. I know this because I almost hit her with my jeep one morning. I was pulling out of a parking lot, and she was coming the wrong direction down the sidewalk, moving MUCH faster than anything on the sidewalk should, and generally acting careless.
It has been pointed out already, but the segways don't really "fit" anywhere. They're too slow for the road, they're too fast for the sidewalk, and they're too big for the bike lanes.
They're well suited for malls and airports, but I can't really imagine myself using them anywhere else.
It would be easy to get around on a Segway, but nobody does it.
They're too expensive. People think (I say unfairly) that they look dopey. And there isn't a group of people that feel it's made for them.
Every time I see one though, I just think of GOB in Arrested Development and bust up laughing...