It was in large part due to competing land claims. In the 1700s, both NH and NY thought that the land between the Connecticut river and the Hudson/Lake Champlain was theirs, at least as far as handing out land grants - a pretty lucrative trade - was concerned. This was not helped, so to speak, by the British crown whose grasp of geography was not great ( nobody's was at the time, although the French and Indian War had helped).

As the Revolutionary War started, the question of the legal status of the State was not a high priority, although Vermonters did participate in the Battle of Bennington on the side of the Americans. Even so, Vermont adopted a constitution which was fairly progressive for the times, banning slavery and eliminating the property qualification for voting.

Still, NY was opposed to admitting Vermont because they felt their land granting rights had been ignored.

Eventually, Vermont paid 30,000 Spanish milled dollars to NY, and their objection was removed.

Further reading:

https://www.history.com/this-day-in-history/new-connecticut-vermont-declares-independence

http://freedomandunity.org/new_frontier/frontier.html

https://ancestralfindings.com/vermont/

Additional fun things: Ethan Allen rowed across Lake Champlain with a bunch of his homies and captured Fort Ticonderoga. The cannons were sent down to Boston to help the fight there at a time when the revolution had very few cannons.

Ethan Allen and his brother Ira held quite a bit of NH land grants, and were accused of driving off holders of competing NY land grants. Ethan was by report quite the young thug when doing so. He and a band of others drove off, sometimes beat up, and often burned out settlers who had NY land grants.

Ethan Allen claims he told the commander of the Ticonderoga fort to "Surrender in the name of the Great Jehovah and the Continental Congress". Since Ethan was agnostic at best and an atheist possibly, the competing narrative is the he said, "Come on out of there, you old bastard".