Presumably Vonnegut kept the draft, or kept the original and sent McCarthy the one copy. The letter's text was later included in Vonnegut's book, Palm Sunday, as mentioned on the site.
Although I can understand your not enjoying this particular letter or agreeing with its inclusion on a blog named Letters of Note, to hear you then use that as a stick with which to beat my entire website - a website you admittedly know nothing about - does smart a little. I'd suggest maybe having a poke around; you may come to the conclusion that today's letter simply wasn't to your liking and that, actually, the website does have some value.