How it happened:

The early drafts of what became the National Firearms Act of 1934 included all handguns, as well as all "concealable" firearms.

The then-attorney general opined that that (a) would be unconstitutional, and (b) would have a huge negative backlash from American citizens.

So the proposal was rewritten to delete handguns ... but "concealable" rifles and shotguns were left in the later drafts, which Congress passed and which became law in 1934.

As far as "why," it's because the U.S. Congress in the 1930's intended to disarm all U.S. citizens, much like has been done in other countries. It is not right, it is not moral, and it definitely is not constitutional ... but it is what it is.

Do not look for logic or morality in the arguments made by fools.