They must have thought words must be, as how it is spoken, written continuously.
That makes a lot of sense.
I can also understand the motivations for standardizing the left-to-right line: then you don't have to learn to read and write each character foward and backward, and writing from left-to-right would probably be more comfortable---for the righthanded at least.
Do you suppose that scripts written from right-to-left originated with lefthanded scribes? Or just by chance? Or another reason?