Why was the government in Massachusetts Bay Colony the most radical in colonial America?
It wasn't. But if you want to make an argument for it: because they were supposed to have governors elected by the "freemen."
The Massachusetts Bay Colony was formed in 1628, but mostly built up in the 1630s during the Great Migration of Puritans from England. The goal of the colony was to build a "City on a Hill." This meant that they intended to build an ideal example of how to live according to God's law, such that the Protestants in England would follow suit.
The Massachusetts Bay Company was technically a company that ran on a charter (granted by the British government). The charter is often cited as evidence that it was the first representative democracy. In the charter, it is states that "freemen" (property holding, wealthy, men of the church) would be able to participate in electing officers. However, this never truly happened.
The first meeting included only 8 freemen, who seized the opportunity to change the rules of the charter. Instead, the 8 formed a Council of Assistants who elected officials instead. This continued for many years, until finally people demanded to see the original charter. Following arguments, they reached a compromise that each town could send two deputies to be the general court (rather than have an open vote). The Council then stripped the general court of most of its powers.
Because the populace eventually forced the council to allow towns to send deputies, the Massachusetts Bay Colony was technically the first representative democracy (excluding all people who were not white, male, property owners, and members of the churches approved by the Council).
Meanwhile, Rhode Island had begun practicing incredibly radical ideas of government, including mandated religious freedom, separation of church and state, and a representative democracy (though it was also mostly limited to upper-class men). While the colony was established in 1636, it did not receive a charter from Britain until 1644.