Uber has "asked forgiveness rather than asking permission" - they've made themselves indispensable in the markets they serve, and it's politically risky to shut them down, because they are serving a very real need in the transportation industry, and people don't want to see them go away. I think that Uber correctly recognized that the taxi market was under regulatory capture, and that they weren't going to be able to break into the market through legal channels, so they took a ballsy risk and decided to give it a shot anyhow.

It seems to be paying off. Massachusetts, South Carolina, Texas, and Virginia have all reversed decisions which banned Uber in their jurisdictions.