The fear of spending 6 months in jail for unsafe storage of weapons will force owners to safely store their weapons.
We have to start somewhere. Why not with safe storage and prosecute those who dont.
The question is, how would you enforce that?
It would involve the police going round to regularly check-up and search the house/gun-storage area of every registered gun owner in their area. Not only would it involve a lot of police time and resources, but I imagine that is the kind of snooping that many Americans, and especially gun-nuts, would hate. It could even turn potentially dangerous if the police attempted to confiscate un-safely stored weaponry.
Especially in areas with a very high proportion of gun-owners which are presumably also the most radically pro-gun areas, not only would it require an excessive amount of police time, but there is always the danger of at least one of the armed "this is my rights" gun-nuts not taking it too well.
There is also the whole Federal/State barrier. There are certain states that I am sure would vehmently oppose any such Federal attempts to enforce a nation-wide law on the matter.