I do believe the etymology in this case lies with Old English 'domes dæg' which was thought to be some apocalypse 6,000 years after creation, which was thought

I do believe the etymology in this case lies with Old English 'domes dæg' which was thought to be some apocalypse 6,000 years after creation, which was thought to be ca. 5,200BCE. As for your question why, I cannot reasonably guess. :-) Somewhat related, you may be interested to read up about the Doomsday Book.