Thats pretty much a simple answer for it.
The way Ive heard it is basically a cyclical calender, so it just actually repeats after 2012.
And then they got slaughtered by the Spainiards and small pox. SO they could nto make the next version of the calender.
So it is all fear mongering based on misunderstanding.
IT may essentially be the same, but miniscule astronomical effects were taken into consideration for each cycle.
Also, the Mayan empire had been in decline when the Spanish first set foot in the new world. The natives they killed with small pox were small tribes, remnants of that civilization, and had merged with other meso american cultures. The biggest threat to Spain's colonization at the time of the conquest of teh New World were the Aztec. Whose people died of causes such as casualties of war, famine, and diseases such as smallpox.
Also, the calender was never a propehcy, that concept came straight out of Hollywood.