Because the legislature there thinks that there’s no such thing as too much gun control:

Speaker of the House Robert DeLeo today unveiled sweeping legislation he said will give the Bay State the “most effective” gun laws in the country in a long-awaited roll-out spurred by the horrific Newtown school shootings 18 months ago.

The bill, called the state’s most comprehensive since 1998 by House leaders, adds the state to a nationwide criminal background database and, for the first time, allows local police chiefs discretion in licensing owners for shotguns and rifles. The legislation also bans the private sale of guns without a licensed gun dealer and requires gun owners to list all the guns they own each time they renew their license.

Other than that, and all the existing extremist gun control laws already in place, Massachusetts fully supports the Second Amendment.