There is little logic or reason involved, but the argument is that a person doesn't need that capacity in a normal situation so the only reason to have it is to go on some sort of shooting rampage and not having it means they can't. Most of the laws regarding firearms are completely arbitrary and are far more of a hindrance to normal people than those intending to break the law.
Side note: You cannot legally purchase a handgun in any state other than your state of residence, but you can buy any other type of firearm. Considering that you go through the same *federal* background checks and you still have to follow the laws of whatever state the firearm is in the law makes zero sense...but that is firearms law for you.