I would explain that it's a painfully tiny sample size, as more than 10 American writers publish every single week day. Even in the 19th century, more than 10 were published every month if not week.
Your sample doesn't include-- if we want to strictly stay with white Easterners --Bronson and Louisa May Alcott, Thoreau, and (IIRC that he was from the Eastern US), Horatio Alger. This is just people I can name off the top of my head.
It also doesn't include obscure authors who didn't stand the test of time.
Oh, and I believe Harriet Beecher Stowe also fits my limited criteria.
Like virtually every kind of conspiracy theory (they tend to have similar thinking behind them), it's a tiny sample size which fits a few arcane criteria, and ignores any counter examples.