This YouTube video from Talking Feds with Harry Litman addresses it (ignore the ridiculous clickbait video title - no-one was shocked by the ruling): https://www.youtube.com/watch?v=sO3c1tOS4Ho
To summarize, there is nothing in the New York State Constitution that states that voting must be in person, but there was a long tradition that interpreted the state's constitution to mean that, and Elise Stefanik made the argument that it did. The court said it was a difficult case, and one justice did vote that it is unconstitutional to allow everyone to vote early by mail.
This YouTube video from Talking Feds with Harry Litman addresses it (ignore the ridiculous clickbait video title - no-one was shocked by the ruling): https://www.youtube.com/watch?v=sO3c1tOS4Ho
To summarize, there is nothing in the New York State Constitution that states that voting must be in person, but there was a long tradition that interpreted the state's constitution to mean that, and Elise Stefanik made the argument that it did. The court said it was a difficult case, and one justice did vote that it is unconstitutional to allow everyone to vote early by mail.