> Your hand is likely to smudge R-L systems unless you're left handed which most people are not
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.
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.