And we don't even understand what the purpose was of the Mayan calendar "ending" on that day anyway
I like to think of it as one of the first attempts at trolling.
Those guys must have thought about what people would think when it was getting near the end of the calendar, so they intentionally kept the dates going until some arbitrary date (21/12/2012) just to trick the people of that time into worrying. =P
That or it was a case of the guy writing out the dates just got bored, went to the leader and said "Look, let's be honest here, I've written out the dates up to XYZ and I really don't think we, as a community of people, are going to last that long. If you look at our history we've just been going around clubbing each other over the head or strapping each other to stones and inserting sharp metal things in them... We're not going to make it this long. Can I stop writing these out now?"
Leader: "I suppose you're right. Yeh, why not."
Those are my best guesses.