because the pre-existing right was shaped by the English common law which generally forbade "dangerous and unusual" arms
you have to define the arms that are protected by the Second Amendment in some way, so the courts have basically done this so far:
1. the Second Amendment extends prima facie to all bearable arms;
2. but not "dangerous and unusual arms";
3. handguns are commonly kept for lawful purposes and cannot be banned;
4. assault weapons are commonly kept for lawful purposes and can be banned;
5. there exist all sorts of limitations on the right to keep and bear arms, such as sensitive places, carry outside the home, prohibitions on felons, violent misdemeanants, domestic abusers, and those adjudicated mentally incompetent
So generally speaking (with assault weapons and large-capacity magazines being an outlier) if the weapon is typically possessed for lawful purposes, it cannot be banned. It obviously matters how people use things. If everybody used their cars to drive into parades, we'd think of them not as vehicles but as weapons.