Because they lived in the mountains and built no boats?
They ended up building an empire based on an area in the mountains, but little is known about their origins. It seems they originated in Northwestern Mexico and were a nomad people (see this
PBS article for example) who eventually settled in Tenochtitlan - where modern day Mexico city is - and surrounding areas.
Given the cultutal level of the civilisations they were surrounded by and eventually conquered I would suggest the origins of their calender and/or timekeeping long predates when they settled in the middle of Mexico. Maybe they just nicked it from someone they encountered in their ancient history.
Who knows where they derived these things from - I'm just saying the mountains bit may be a red herring.