There's also that catch-22. Machine guns are banned because they are not in common use, and they are not in common use because they are banned.
Although if the registry were open, and the price of MGs returned to their normal market value, then yes, I believe they would be much more common. I mean why buy a semi-auto-only rifle if you can have a select fire version for like $50-$100 more. Even if you won't use full-auto often because of ammo costs. Might as well have the option available to you. If the registry were gotten rid of altogether then I doubt manufacturers would even bother making semi-auto only versions. It would be the death of the AR-15