The Bill of Rights was approved by Congress in September 1789 and sent to the States for ratification, a year after the US constitution came into force.
150 years later, there was a sesquicentennial celebration of the Bill of Rights, and it was pointed out to the three states that they had still had not ratified. So they did so, principally in order to be part of the celebrations.
Massachusetts and Connecticut legislatures had essentially voted to ratify all or most of the articles originally (remember that there were initially twelve, of which only ten were ratified quickly) but different numbers of in each chamber, and had never bothered to reconcile this position since it made no practical difference after other states had ratified. Georgia had rejected the need for a Bill of Rights in principle in the 18th century (some argued that the Constitution should be limited to what the United States was explicitly positively allowed to do, and by including restrictions it gave the impression that the federal authorities could otherwise do anything which was not prohibited) but all those politicians were dead by the 20th century.