A lunar month is inconvenient. It does not divide the year, it does not divide the week, and it forces leap months on anyone who wants both. Almost every early civilisation used one anyway.

Because it is the only unit of time you can check. A year has to be measured against a horizon over decades; a month is visible from any doorway, and any two people can agree on what they are looking at without instruments, writing or trust. Every tradition in this file rests on that: the Moon was the first public clock, and it was free.