It might be. I’m not up on the history of NYC roadways.
Again the point being made is not why “the” versus "no ‘the’ ". IMO that’s an unanswerable question about the why of a random dice roll of local cultural history.
The point is how “the” came to be used with route numbers. And the answer is that “the” was already in use before route numbers became popular ways to define roadways.