Simple!
Maura Healy is a political animal. She's also smart and very ambitious. She does not do anything unless it improves her political standing.
She was never interested in "upholding the law" or banning guns. The only reason she made those "proclamations" in 2015 is because she wanted to get the job of the head of the Justice Department in HRC's cabinet, once she became the president. When HRC bombed, instead of going down with the ship "HMS Hillary", Maura regrouped and started positioning herself for the gubernatorial job. Historically, state governors have far better chances of becoming president of the United States than any other position(Bush Jr., Clinton, Reagan, Carter, Nixon, etc.)
The other reason why her office never prosecuted anyone using the "new" interpretation is that it would result in a lawsuit that would overturn MA AWB which would be the mark of death on her political career. No prosecutor wants to have their name be on a case that goes to the SCOTUS and loses.