Why was the word moor printed instead of using words from the newspaper like the rest of the message?

The word moor was printed because it wasn't found in the paper.

“With gum on to the paper. But I want to know why the word ‘moor’ should have been written?”

“Because he could not find it in print. The other words were all simple and might be found in any issue, but ‘moor’ would be less common.”

“Why, of course, that would explain it. Have you read anything else in this message, Mr. Holmes?”