Why are there six regions in the United stated?

Answer 1

http://memory.loc.gov/ammem/gmdhtml/rrhtml/regions4.html

Northeastern States (Mid Atlantic region plus New England) East of Mississippi River, North of Ohio and Potomac Rivers, North of Mason-Dixon Line:
Connecticut, Delaware, District of Columbia, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont,

Middle WestEast of the Rocky Mountains to the Allegheny Mountains, North of Ohio River and southern border of Missouri and Kansas, North of Mason-Dixon Line:
Illinois, Iowa, Indiana, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, Wisconsin

Northwestern States Northern part of US between Rocky Mountains and Pacific Ocean:
Montana, Nebraska, North Dakota, Oregon, Washington, Wyoming, Idaho

Southern States South of Mason-Dixon Line and Ohio River, West Texas border to the Atlantic Ocean:
Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, , South Carolina, Tennessee, Virginia, West Virginia, Oklahoma, Texas,

Southwestern States Southern US West of Mississippi River:
Arizona, California, Colorado, Nevada, New Mexico, Utah

Pacific States States who border the pacific ocean and no other US land mass:
Alaska, Hawaii