Why 9-1-1 can't locate you on your cell phone

FAQ: 
Why can't 9-1-1 locate me on my cell phone?
Answer: 

Simply, the technology does not exist from cell phone providers. Dispatchers are able to get the location of the cell tower you've connected to. In some cases, a "wireless phase 2" location is received from the cell tower, which provides a more accurate location of where a caller may be calling from. However, this varies from cell provider to cell provider, location to location, and cell phone to cell phone.

Always be prepared to give 911 dispatchers your location, even when travelling.

For more information, take a look at this article, which goes more in-depth.