Because Vermont was not a colony, but a disputed territory. It had been claimed by Massachusetts, New York and New.Hampshire. In 1764 the King assigned Vermont to New York: even though a number of townships there had been already surveyed and founded by New Hampshire. Massachusetts gave up its claim, but there was soon a conflict, with Ethan Allen and his Green Mountain Boys wanting a new state, the border to be at the Hudson River. That resulted in a 1777 convention, called the Westminster Convention, to create a new state. But all disputes were set aside for the Revolutionary War, when all involved wanted to fight the British.

Vermont after 1781 became its own country, waiting to join the United States. But because of those land claims against NY, the conflict between them grew almost to a war, with heated rhetoric and militias marching around. And, that border dispute prevented VT from being admitted as a state. It was eventually decided to have the border set by the new federal government. That happened in 1790: NY paid Vermont for the NY territory now east of the Hudson, and VT joined the US.