Concerning the final paragraph of your comment in those comments, the wide distribution of the sermon can be considered a manifestation of general interest in all matters concerning what was effectively the world war of the eighteenth century. This, however, does not explain the specific interest in a Jewish sermon. Only in the final 1763 publication of the sermon in America is there a hint as to what made this sermon so popular, i.e., the increasingly pressing question of how the Jews fit into a newly-emerging modern society. The publisher exhibited an enlightenment mentality in two regards. First, he lamented the manner in which Christians have abused the Jews and he hoped for an end to this. (This, of course he hoped, would engender an environment more conducive to the conversion of the Jews.) Secondly, the sermons illustrated that “they have patriot[ic] sentiments, and the warmest gratitude to princes who have wisdom and humanity to protect and defend them.” I.e., if the Jews are treated well they would cease to restrict themselves to the margins of society and instead wholeheartedly identify with the larger nation. That it was an American publisher in particular who gave voice to the enlightened sentiments behind the repeated publication of the sermon foreshadowed the welcome reception that Jews were ultimately to find in the United States. See the publisher's postscript. I think Freudenthal dealth with this. Chag samei'ach.
While this is true, as I said, if you look at the periodical literature which review the sermon (before 1763), they all occasion surprise and interest at something so "normal" penned by a rabbi. So I would argue that even earlier than the publication in America, even earlier than a publisher writing an introduction, you see hints as to why it was popular in Enlightened circles. See a similar thing, by the way, some 50 years later when sermons by Solomon Hirschel were similarly lauded for being so normal.
References to On the Main line and King of Prussia are to Philadelphia-centric for me :-)http://www.simon.com/mall/king-of-prussia-mall/stores-LFD
Interesting to note; it seems from here http://hebrewbooks.org/pdfpager.aspx?req=1697&st=&pgnum=242&hilite= (ad she'ba) that the Korban HaEdah's father was well acquainted with the "Glorious King of Prussia".