Watching John Adams on HBO Max and it's pretty good although am curious why Maine and Vermont weren't included as part of the original 13 colonies. On the flag the marchers held through Boston to protest "join or die" all of New England is included as one part of the snake without differentiating the New England states.
Were Maine and Vermont not included because they at that point were more part of France or Canada?
The premise of your question is not correct. Both were part of the original 13 colonies.
Vermont was under dispute between New Hampshire and New York, both claimed the land between the Connecticut River and the current border with NY. That was settled in 1791 with the disputed area admitted as its own state.
Until 1820 Maine was part of the state of Massachusetts.