The governments in most other countries manage to enforce registration and i am sure America could do if they wanted to.
Actually, enforcement in the UK and Australia has been pathetic, with around 20% compliance rates.
The thing is, you don't have to go door-to-door to enforce such a thing. You just have to bust a few people now and then to have a chilling effect. Jim Bob can't find the paperwork for that gun he's shooting at the range? Easy bust. Single inner-city mom shoots an intruder with an unregistered gun? Easy bust.
Journalist gets caught waving one around on the air? Oops. Prosecution wouldn't serve the public interest. Expect enforcement to be very selective and inherently unfair.