I thought maybe to account for the origin of italics, it could be that it's easier to slant in that direction while writing left-to-right than the other way.

However, when people write Yiddish (which has a right-to-left script) they slant the same way[^1], whereas you'd expect the opposite if it were easier, so maybe it is just arbitrary.

[^1]: An example of Yiddish handwriting https://en.wikipedia.org/wiki/Yiddish_orthography#/media/Fil...