Part of it has to do with indigenous maritime roots of Anglo American civilization. Active thriving coastal communities such as Y'ha Nthlei and Ahu Y'loa, along with their shore side antecedents of Innsmouth and Dunwich (UK) have ensure a strong popularity for the Lord Beneath the Waves, along with his lieutenants Dagon and Hydra. The Esoteric Order of Dagon has long organized food and monetary charity (fishes and golden idols), as well as provided a valuable match making service in coastal communities.

By contrast, the Hermetic Order of the Shining Twilight, like the Starry Wisdom Church, has been exclusive and elitist. Cthulhu worship is more open and accepting of racial and cultural diversity.

Of course, Shub-Niggurath has always been popular with agricultural societies (citation: Campbell, Ramsay), Mordiggian is mostly popular with Hipster ghoul cults of bohemian artists, as he's pretty underground.

I've long had this pet theory that it has to do with the fact that Cthulhu, being some sort of Cephalopod Old One, is smarter than the average eldritch entity (cephalopods are smart) and so is merely better at organization and recruitment.

However, I'm not an expert in the field. Perhaps somebody from Miskatonic is currently here?

Jdiejejskdjrjdjsiwjdjdjej, sosmwa dke'soo kakakakakweeee! AIIIIEEEEEEEEE!

(Source: I am the Squid Faced God by Cthulhu Fhgtagen)

This is all a smoke screen operation by the esoteric order of Dagon. I will not deny that Cthulhu has become a popular figure, the merchandising alone is proof of that, but the spread of the Cthulhu mythos was largely the work of a cabal of EOD leaders. A number of academics, private investigators, and people whose cars had broken down in small beach towns had all vanished while investigating the EOD, and they needed someone else to pin the blame on. In comes the largely insane cult of Cthulhu, a natural news maker, and before long everyone is blaming everything on a someone who isn't even awake all while the EOD continues to control the minds of fish.