They're higher quality because they're curated. It's possible that the average submission is better than the average internet comment due to the level of effor

They're higher quality because they're curated. It's possible that the average submission is better than the average internet comment due to the level of effort required. Or perhaps some of it is due to the fact that the printed letters are archived for posterity. But regardless, the average letter is not printed. There is only room for a select few.