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Well, see. The end of the world is something so large, so important that we cannot possibly ignore. And no matter who says it, they appeal to and hold merit to some specific audience. They're definitely not random, as they are calculated, albeit with fallacies and pseudo-science, but calculated nonetheless from old texts and all that. Hardcore Christians will believe in the rapture, because according to some it is calculated from the Bible. Mayan cult followers will believe in a catastrophic significance in 12/21/2012 because it is, according to some, calculated from Mayan texts. Of course, nobody else believes that these dates will be of any cosmic significance unless completely by coincidence, but as long as there are cults who are plastering their opinionated facts on the internet, rioting, and killing themselves, we are bound to notice and repeat this argument on why it is all bullcrap and why we still go through the process.

TL;DR: They are always a big deal because the dates aren't random, but appeal to the beliefs of a specific audience.

Some doomsday theories, however, are entirely possible and factual. There was the aforementioned Y2K and 2038 Unix time disaster, and then there's the possibility of a nuclear holocaust, or a date based on estimates as when global warming will become uncontrollable and kill us all.