Because it would require a constitutional amendment. That requires a proposal signed on to by 2/3 majority vote in BOTH the House and the Senate, or by a constitutional convention called for by two-thirds of the State legislatures. Once the amendment is successfully proposed it would then have to be ratified by 3/4 of all state governments. Gun grabbing Democrats only hold a majority in 18/50 state legislatures. It's not supposed to be easy to change the Constitution. Gun "nuts" understand that freedom comes with risk and there's no such thing as security. Everybody dies. Life is tough. It's tougher when you depend solely on uniformed strangers to protect you and your family.