Interesting comments with most bearing merit.
Sometimes design engineers use an algorithm dependent on some factor(s). Cost, redundancy, frequency of cycle, lowering transmission losses with increased potential.
Perhaps the general availability of GC batteries can keep cost down.
A parallel configuration allows one to use one battery in an array to be used in limp mode should another battery (cell) fail. Series, not so much.
Ever been in a POTS Plain old telephone system exchange? Large rooms of commercial heavy wet cell batteries wired in parallel doing little or nothing just in case they might be required to provide 48V for phone service in an outage. May only happen once or twice in their entire lifetime.
GC golf cart batteries wired in series, cause they are relied upon to be taken to a low state of charge, generally daily. Frequent cycling.
High end coach or tactical equipment might have parallel cells wired with ability to be hot swapped, or have redundancy at a higher cost.
Some maintenance free, spill proof, you bet. AGM, LiFePO4 (Lithium) ...$$$ Check out aircraft arrays.
Higher voltages can allow lighter gauge wiring and switch / fuse gear. Perhaps less line loss.
Ultimately many arrays are a combination of multiple configurations.
I'm waiting to be able to afford a LiFePO4 array, half the weight twice the wattage. Sigh!
DRV Suites ES-38RSSA #9679
GM Denali 3500HD-Max 4x, CC, 8' DRW
EZGo-TXT, Yamaha-G22E, CC Precedent