I haven't read the CRB cover to cover, but so far I haven't seen anything that explains the logic behind that choice. Anybody have insight?
Starfinder is a derivative of the D20 system used in D&D 3.0 and D&D 3.5 It is itself a very close evolution of Pathfinder, which was directly based on ver. 3.5 of D&D.
For for more than 16 years we have been playing with a 5' square grid. The entire gamer culture, experience, VTT systems and electronic mapping aids are based on it. That system, in turn dates back to earlier versions of AD&D and to Original D&D going back to 1974.
It was always a graph paper game, after all.
The experience of the player base with hexes is small. Hexes do not work well indoors at all. Whether that is inside a space station, a temple or inside a starship, they are particularly ill suited to the task of mapping right angled geometry. Humans build in more or less straight lines, not hexes. This is not controversial and has always been why hexes are poorly suited to RPG use since Gary rattled a pair of polyhedral dice standing over his basement sand table.
That Starfinder has hexes for starship combat at all is a largely a design decision of the lead designer, Jason Bulmahn, who is himself an old fan of Star Fleet Battles. Starship combat is an element of Starfinder which is his love letter to SFB. But for that, there is a very good chance we'd be playing Starship combat on squares, too, and using diagonal directions within a square to provide 8 turn points/facings and not the mere six which hexes limit us to.