Cults of Cthulhu 

Despite his fame and influence on Earth and the development of humanity, The Great Cthulhu is really just a minor power in the Mythos, as he's merely a Great Old One. Great Old Ones are incomprehensibly powerful alien beings, but they are still just mortal beings (though beyond what humans can consider "mortal.") It is totally conceivable that a Human could rise up to the point of becoming a Great Old One, basically becoming the GOO representative of our species (think the Elders of the Universe in Marvel.) Nyarlathotep is an Outer God, and the Outer Gods are actual _Gods_ and embodiments of cosmic principles (like Yog-Sothoth is space and time, Shub-Niggurath is life/birth/reproduction/death, and Azathoth is creation and destruction.) Nyarlathotep is the Messenger and _Soul_ of the Outer Gods, being basically the only one with an actual mind, and is far, _far_ more powerful than Cthulhu (Cthulhu himself is _just the High Priest of Yog-Sothoth_ for his species. _Cthulhu worships the Outer Gods.)_ Nyarlathotep is so prevalent because if you want to worship the Outer Gods, then you _have_ to deal with Nyarlathotep. He's the Messenger and the Mouth of the Outer Gods. The rest are basically just cosmic principles with urges. To continue the Marvel comparison, I'd put Cthulhu at about the level of The Silver Surfer, and Nyarlathotep at the level of Galactus, maybe? Enough of a difference that Nyarlathotep could easily snuff Cthulhu if he wanted to.