Just a wild guess, but I would think it is an artifact of the US
sectional/township system. Most property lines are along sectional lines (or
quarters of sectional lines) so runway length may be determined by what is
the longest diagonal that will fit on a square mile.

Either that or there is some FAA reg that involves runways of greater than
5000' that the designers wanted to take advantage of. But even there the reg
probably is based on the biggest runway you can fit on a section.