It's an inconvenience, basically. I think it comes from paranoia and this propagandized fear of the government and conspiracy theories. Not to mention, people like to do what they want to do, when and how they want to do it. Registering for guns and requiring background checks unilaterally would interfere with that. It also probably has to do with the fact that it's all a way to crack down on criminals and gun-owners appear to be easily offended over the idea that they would be subjected to the same laws that a criminal breaks (or circumvents) in order to commit their crime (though they handle the rest of the law just fine). But I would say a lot of it does fall back on fear.
The Germany/Hitler-took-all-the-guns myth, for example. People use that as an excuse for why the idea of gun control is bad. They compare Obama to Hitler and shout 'communism!' and 'Nazis!'.
Hitler only confiscated guns from the Jews (for reasons that are obvious to history and were very, very racially motivated). The government prior to Hitler and the Nazis had already put gun registration into effect (the Nazis actually eased up on it - not to mention, those laws had been put into place to try and limit gun ownership of Nazis in the first place, so once the Nazis took over, they were able to use those gun laws to their advantage). So Hitler, Germany, and their gun registration/ownership issues are an absolute non-issue when it comes to the gun control situation here in America. Not only were they very different times, but the circumstances, motivations, and politics were all different as well.