Because of ignorant arguments like the following from one Rees Lloyd which permeate Christian Nation circles. These myths have been circulating since Parson Weems made up out of whole cloth stories about Washington's piety. Scholars who doubt Washington's orthodoxy have exposed the stories that supposedly show Washington's Christianity to be false. Lillback understands this dynamic and here is how he explained it in his last article:
Moreover, historians on all sides of this debate over Washington’s true faith would agree that the sheer greatness of Washington makes him liable to hagiography and exaggeration. The unsubstantiated legends of a previous era had to be subjected to the rigorous canons of critical historiography. While some of the testimony for Washington’s faith falls in the arena of unsupportable legend, there is a temptation simply to dismiss all evidence of his faith by assuming that there is only hagiographical and apocryphal testimony to support it. So self-evident did Washington’s Christian faith seem to prior generations, that they only slightly felt the need to establish a scholarly case. Thus when this earlier case for Washington’s Christian faith was examined under the microscope of serious scholarship, it was unable to withstand the assault.
So Dr. Lillback, as we are about to see, obviously doesn't need the likes of Rees Lloyd defending his work because Lloyd's "evidence" for Washington's orthodoxy is exactly what collapsed when serious scholars examined the evidence under the historical microscope. That, plus Mr. Lloyd engages in ad hominen insults that are completely off the mark.
It has been the conventional wisdom in contemporary academia – dominated by now-aging 1960s Marxists and radicals who fled to the universities when their dreams of being the "vanguard of the revolution" turned out to be (literally) pipe dreams that went up in smoke (also literally) – that Washington, as claimed by historian Paul F. Boller in the leading tome, "Washington & Religion," was not a Christian but was, rather, merely a deist who mouthed Christianity for political purposes.
What Boller actually points out, accurately I might add, is that Washington almost never identified himself as a Christian or spoke in Trinitarian Christian terms. Washington supported Christianity in particular because he supported religion in general. And Boller never claims, from my memory, that this support for "religion" was "political" as opposed to heartfelt.
Lloyd goes on:
This has been the cant of liberals in academia, notwithstanding  the many, many statements of Washington himself to the contrary,  as well as his conduct as an Anglican vestryman in his church,  his writing of a personal prayer book,  his collection of sermons,  his conduct of biblical studies with his family,  his appointment of chaplains in the revolutionary army,  his commands that his soldiers attend worship,  his putting his hand on the Bible when taking the oath as president,  his declarations of days of "Thanksgiving" as president,  his crediting "Providence" for his own survival in war and for the success of the American republic, and,  among other things, the fact that he was believed , by his contemporaries, to be a Christian and a man of profound religious conviction.
 Perhaps the most moving image in the American iconography is George Washington kneeling in the bloodstained snow of Valley Forge, praying. He was to say to the soldiers, who were farmers, workers, ordinary Americans fighting, suffering and dying for freedom in the revolution – when never more than one-third of Americans supported the revolutionary war: "The fate of unborn millions now depends, under God, on the courage of this army. Our cruel and unrelenting enemy leaves us only the choice of brave resistance, or the most abject submission. We have therefore to resolve to conquer or die."
And to respond to each of these claims. The bracketed numbers above are mine.
1] While many of Washington's statements show he was a devout monotheist, rarely did he speak in explicitly "Christian" language. 2] The vestryman position was far more political than religious. Thomas Jefferson and many other elite Virginians who didn't believe in orthodox Christianity also served as vestrymen for the Anglican Church. 3] George Washington wrote no personal prayerbook; this has been debunked as a fraud. 4] Pious clergy sent Washington their sermons for him to read and he didn't throw them away but stashed them in his library next to the works of "infidels" like Thomas Paine and Joseph Priestly; this hardly shows Washington was orthodox. 5] While Washington, as a child, may have received a "Biblical education" and studied with his devout mother, no evidence shows that the adult Washington engaged in "Biblical studies" with his family; this is another Christian Nation myth. 6] He appointed Chaplains in the military much for the same reason why we have Chaplains today: to meet the needs of soldiers. Even Michael Newdow and Barry Lynn, as I understand, support military Chaplains for this reason. 7] True Washington did command his soldiers to attend religious services, but again, it hardly shows Washington to be orthodox, rather that he wanted his soldiers to be "religious" because he thought "religion" was necessary for morality and character. However, like Adams and the other key Founders, he likely thought all of the exotic world religions of which he was aware could serve this function. 8] Washington put his hand on a Freemasonic Bible when taking his oath; Freemasonry is not generally associated with orthodox Christianity; 9] and 10] perfectly illustrate Washington's generic supplications to God and only show that Washington was a devout theist, not an orthodox Christian. 11] About half of his contemporaries believed Washington was a devout Christian, and the other half believed he was either a Deist or not a "real Christian" in the orthodox Trinitarian sense. The half that didn't believe Washington was an orthodox Christian included Jefferson, Madison, G. Morris, three of Washington's own ministers, and some pious figures who knew Washington personally, like the Rev. Samuel Miller, but who unlike Parson Weems didn't see the "evidence" for Washington's orthodoxy.
12] And this too has been debunked as a fraud. The Isaac Potts story has no foundation in the historical record and Washington, when he prayed, was not known to have kneeled.
After seeing that the evidence for Washington's orthodoxy simply cannot be gleaned from the historical record and even after seeing everything that Lillback offers in his 1200 page book, Paul F. Boller's summation of the matter still applies:
[I]f to believe in the divinity and resurrection of Christ and his atonement for the sins of man and to participate in the sacrament of the Lord's Supper are requisites for the Christian faith, then Washington, on the evidence which we have examined, can hardly be considered a Christian, except in the most nominal sense.
Finally, it's not surprising that Mr. Lloyd would peddle so many pious frauds about Washington. Among those works he cites for Washington's orthodoxy are: "William J. Federer's indispensable 'America's God & Country; Encyclopedia of Quotations." This book is one of the sources for almost all of David Barton's phony "unconfirmed" quotations which have left the "Christian Nation" crowd with so much egg on their faces.