Why would it be harder? we only find it easy to write left to right because this is all we have known/used. if we was educated writing right to left we would be used to it.

It is harder all right, as you can see if you watch the various attitudes that left-handed people adopt when writing, especially when writing in liquid ink, that easily blots or smears if you touch it before it gets dry. We see less of that nowadays because of the nearly universal use of volatile inks, such as in felt tip pens, and inks that soak in almost immediately, such as ballpoint inks. Right-handed writers have their hand clear of the paper where the writing is to happen, but a left-hander's hand passes over the surface where he has just written.

It also is easier for right-handers because they can see not only what is happening under the pen tip, but the last few letters of what they have written, which are hidden by the hand of a left-handed writer.

Apart from writing from left to right or right to left or, for that matter boustrophedon, in which one alternates (an attractive idea, I always thought!) at least some common scripts go from top to bottom; I do not know how many other variations one gets. I have a few ideas for better schemes, but I am sure that everyone has far too much seems even to consider them, given the preponderance of right-handers and the dominance of tradition and custom. There is a huge number of possible variations, including spirals and helices, not to mention the scytale and similar transposition encryptions!  :-)

My personal impression is that each of these conventions originated, incompletely considered, with the whims and preferences of the first calligraphers in each script. They, after all, had no hindsight to warn them of the unfortunate consequences for ease of reading and writing, much less what a huge volume of reading and writing would follow according to the customs that they were instituting.

@ jon, I appreciate your point that it was probably random chance the direction of writing but what about the second part of the point? as the script is already established, do a greater  amount of people write arabic with the left hand than in left to right scripts.

Any statistically significant increase in the proportion of left-handed writers in Arabic is due entirely to the provisions of Sharia Law.

When they discovered writing there was no problem with that - few people could write anyway and they used different tools than we do today, tools like sticks on clay or they carved stone or something.

The truth is that many people in western and also easter contries who would normaly use left hand were pushed to use right hand for writing because that was the "right" way to do it. Left hand should be "left" for other purposes. Even today you can probably find less people writing with left hand then you would expect. It is not something that is completely spontanuos, there is this social even moral component.

I can write with both hands, I can write in both directions (like a mirror reflection of usual text) and I can do it with both hands in the same time either in the same direction or in opposit directions. Most of all I like writing with my left hand and find it the easiest way to write.