Christianity began as a doomsday cult.
Jesus got it wrong; Mark 9.1. Having found a group of believers who don't mind disappointment, those holding the reigns of the cult back then kept the illusion going--it was in their interests to use the "end time hype" to strengthen their powerful superstition. After the Roman church assimilated all the prevailing cults and brought them under the one Catholic umbrella, the church itself became the authority-- not the Bible, its leader the Pope claiming divine right to be the vicar of Christ i.e. his deputy.
However at the Reformation in Europe the scripturalists (my word) arose and broke away from the Catholic faith taking the Bible literally as the word of God. This allowed the "imminent end of the world" party to form again.
The mainspring which drives the power train of Jehovah's Witnesses is "end times hysteria". To keep it going, like all clockwork, it has to be rewound frequently.