So I have been thinking about this lately, and I've been wondering what exactly are the reason for a culture to write in a specific format? Script form (curvy letters, straight letters), writing material (clay, paper) and development of language (developed from pictures, developed from sounds) are all factors that influence the way a language is written. But I know for a fact that languages have other factors that influence the way langauges are written, but what could they be I wonder?
Easy answer: I don't know. I'm not sure anyone else does, either, after reading a few reports. Check this one out.
HOWEVER, after thinking about it a bit, I've an idea.
In the beginning, Man had sticks and clay. It's nice to think that you can write on clay with sticks like you can paper with a pencil, but you can't — not and expect the inscription to last for any appreciable time. The marks need to be deep, and you need to write fast 'cause you're the scribe and the king waits for no serf.
In other words, in the beginning, people had to press to make the letters. Since most people are right-handed it's very natural to press both in toward the body and left, pushing with the hand. Heck if I know if this is why it occured, but at least it's almost logical.
Later, as charcoal came into vogue, the need to press disappeared and the smudging problem everybody thinks about but no authority wants to admit could be an issue comes into play. Think about it... a bazillion years of pressing sticks into clay and suddenly the king expects you to use this new-fangled thing called "the tip of a burnt stick" 'cause he's not in the mood to be less cool than that stink-bomb of a king over there.
And left-to-right is born. Dragging the stylus rather than pressing the stylus.
And the belief that left-handed people are all witches is born, too. 'Cause the king simply can't believe that anyone not born of the devil could fail to write things his way. I'm just sayin'.
It depends on both your writing medium and your writing support.
Both will influence the way the text is arranged and will somehow enforce a standard.
Actually there are examples of languages which were or are written indifferently from left to right or up to down. Japanes and Chinese are some modern examples, and I think also Hebrew was versatile in the past, being written both from right to left and left to right.