Earlier quoted context omitted.
I wonder if the standardization of directions, rather than boustrophedonic writing, is due to the technology of paper and ink. If you're carving into stone or clay, then your hand isn't as likely to smudge the already written text.
I used to think that, but a bit of reflection puts that to bed: 1 - The earliest ink-and-paper writing in the Middle East was in Egypt, where the pen writing mostly followed inscription, being a little free form in ordering. AFAIK the old language didn't acquire an alphabetic system until Greek colonization (cf Coptic, which is (evolved) pharaonic Egyptian written with essentially Greek characters). That latter is in…
Arabic was originally read Right-to-Left, but written Top-to-Bottom, leftwards, on the scroll so smudging by right-handed writers wasn't an issue. A completed scroll was then hung sideways on the wall for people to read. In fact, by writing Arabic T-B(leftwards) instead of L-R(downwards in columns) like Greek was at the time, smudging became less of an issue. The Arabs could roll up the left side of their scroll sooner without smudging or pausing, whereas the Greeks had to leave the entire column they'd just written exposed while the ink on the bottom rows was drying.