let me rephrase that cortez stole their only writing untensils and they forgot how to write

Dont you think a 2000 year calendar is enough? lol

They ran out of fingers and toes and couldn't count any higher.

It didn't. The Mayan calender is cyclical, the ending of one age being the start of another. This is a common mistake that people have tried to exploit to add weight to the 2012 doomsday nonsense.

may be they don't know to count beyond 2012.

They ran out of space on the rock.

The Mayan King had this smart but really obnoxious nephew living at the palace, he had to find a job for but didn't want around. He had this really great idea to make a tribute to himself with an amazing calendar! So he set his nephew creating the calendar on a remote mountain top, the better to see the stars with, and everything was peacefull around the palace again. One day an advancing army came, tearing up his land and killing his people. The King didn't have the army to fight so he met on the battlefield with the advancing Commander. The Commander of the army said "I am the smartest man alive! No one is smarter than I am! I will have a challenge of your smartest man. Send one man out here and we will match wits!" The King ran to his nephew in the mountain and said you have to come and save us! The nephew said "but I've only gotten to the year 2012! The King said no matter, come! So the nephew met the Commander in the battle field and matched wits with him. Storm clouds gathered around the two they battled so hard! When the storm clouds cleared they saw the nephew walking away from the prosrate form of the Commander. Needless to say the nephew was now the King and he forgot about all the calender.