Because Lovecraft (in Laundry-verse) was a fraud whose work was, by and large, not very close to the Truth. Even so, the Laundry and its counterparts in other countries did censor some of his works which, by pure chance, came a little too close to the actual truth.
Stross goes into some detail on the topic in his short story Equoid (fair warning, this story is messed up even by Laundry standards, and also extremely irreverent towards Lovecraft). Important bits bolded by me:
You’re probably thinking WHAT THE HELL, H. P. LOVECRAFT? And wondering why I’m reading his private letters (most certainly not found in any of the collections so lovingly curated by Lovecraft scholars over the years, from August Derleth to S. T. Joshi), in a file so mind-numbingly trivial that its leakage on the front page of a major tabloid newspaper would be greeted with snores.
You probably think HPL was one of ours, or that maybe one of our predecessor agencies bumped him off, or that these letters contain Great & Terrible Mysteries, Secrets, & Eldritch Wisdom of the Ancients and must be handled with asbestos tongs while reading them through welders’ goggles. Right?
Well, you would be wrong. Although it’s not your fault. You’d be wrong for the same reason as the folks who think modern fly-by-wire airliners can fly themselves from takeoff to landing (who needs pilots?), that Saddam really did have weapons of mass destruction (we just didn’t search hard enough), and that the Filler of Stockings who brings presents down the chimney every Newtonmas-eve is a benign and cheery fellow. You’ve been listening to the self-aggrandizing exaggerations of self-promotion artists: respectively, the PR might of the airliner manufacturers, dodgy politicians, and the greeting card industry.
And so it is with old HPL: the very model of an 18th century hipster, born decades too late to be one of the original louche laudanum-addicted romantic poets, and utterly unafraid to bore us by droning on and on about the essential crapness of culture since Edgar Allan Poe, the degeneracy of the modern age, &c. &c. &c.
His reputation has been vastly inflated—out of all proportion—by his followers, who think he is the one true wellspring of wisdom concerning the Elder Gods, the Stars Coming Right, and various hideous horrors with implausible names like Shub-Niggurath, the goat of a thousand young, who spawns mindlessly on the darkest depths of the forest . . .
. . . Whereas, in actual fact, his writings are the occult equivalent of The Anarchist Cookbook.
It’s absolutely true that Lovecraft knew stuff. Somewhere in grandpa’s library he got his hands on the confused rambling inner doctrines of a dozen cults and secret societies. Most of these secrets were arrant nonsense on stilts—admixed with just enough knowledge to be deadly dangerous. Occultists of old, like the alchemists who poisoned themselves with mercury in their enthusiasm to transform lead into gold (meanwhile missing the opportunity to invent the modern discipline of chemistry as we understand it), didn’t know much. What they did know was mostly just enough to guarantee a slow, lingering death from Krantzberg Syndrome (if the Eaters in Night didn’t get them first). Not to mention the fact that the vain exhibitionists who compiled these tomes and grimoires, strung out between the narcissistic urge to self-exposure and their occupational addiction to secrecy, littered their scribbled recipes with booby traps on purpose, just to fuck with unauthorized imitators and prove how ’leet they were for being able to actually make this junk work without melting their own faces.
But the young idiot savant HPL was unaware of the social context of 18th century occultist fandom. So he naively distilled their methanol-contaminated moonshine and nonsense into a heady brew that makes you go blind and then causes your extremities to rot if you actually try to drink it. It’s almost as if he mistook his grandfather’s library for a harmless source of material for fiction, rather than the demented and dangerous documentation of our superstitious forerunners.
The Anarchist Cookbook, with its dangerously flawed bomb formulae, hasn’t maimed half so many hands as HPL’s mythos. His writings look more like fiction than allegorically-described recipes to most people, which is a good thing; but every so often a reader of his more recondite works becomes unhealthily obsessed with the idea of the starry wisdom behind it, starts thinking of it as something real, and then tries to reverse-engineer the design of the pipe bomb he’s describing, not realizing that Quality Control was not his strong point.
There are bits of the True Knowledge scattered throughout HPL’s oeuvre like corn kernels in a turd. But he left stuff out, and he added stuff in, and he embellished and added baroque twiddles and stylistic curlicues as only H. P. Lovecraft could, until it’s pretty much the safest course to discount everything he talks about—like Old Bat-Wings himself, Dread Cthulhu, who dead but dreaming sleeps in Drowned R’lyeh beneath the southern ocean.
Watch my lips: Cthulhu does not exist! And there is no tooth fairy.
(Santa Claus is another matter; but that, as they say, is a file with a different code word . . .)