Undue influence.
In particular, information control. Lovebombing. Harsh punishment for those who do not agree 100%.
And what Slim Boy Fat said about 1914. Had WWI started in another year, some other Adventist group using the 2520 year calculation would have likely used it to confirm their prediction and the Bible Students would have long faded away and been entirely forgotten.
Of course, whether this group would become a fullblown cult like JWs would require a tyrant like Rutherford to take over.