DC is Uber's second largest market outside of San Francisco - and that includes Chicago, NYC, LA, etc. It's a test kitchen for a lot of consumer services because it's got a lot of different demographics under one roof. It's also relatively small, making it easy for Uber drivers to get around the area without incurring a bunch of extra costs for the company. Grocery delivery is also super popular in DC already.

San Francisco, by contrast, is almost too perfect. It would no doubt be successful there, but it doesn't give them as good of a feel for how this service would fare outside of Silicon Valley where these types of ad-hoc delivery services are far more common.