If smartguns become popular, the government can then outlaw non-smartguns without too many people caring (they can still own and fire guns).
Then the smart-stuff is leveraged to "enhance" security - transferring ownership, for an example, could be made subject to a government permit, which can then be used, explicitly or implicitly - imagine a "no fly list" for guns, with the same opacity - to prevent large groups of people to own guns (well, specifically, to fire them).
Further not-very-fantastical "enhancements" could include remote-disabling of guns, initially for law enforcement use, but quickly extended to much broader use.
All this, of course, while the criminals (who are already obtaining guns illegally) keep using "dumb" guns and aren't affected by this.
These are the simple, practical reasons - there's also the ideological reason founded on the historical reason for the 2nd amendment: A government can't turn on it's own people if said people is armed. If the government can effectively control the guns of the people, that stops being a factor.