Why is Arabic written from right to left?

You ask the wrong question. The question to ask is Why is English written from left to right? The Arabic and the English alphabets are both derived from the Phoenician alphabet, written from right to left.

Evolution of Alphabets
The major European alphabets, Latin and Cyrillic, derive from the ancient Greek alphabet. The Greeks adopted their alphabet from the Phoenicians, who used an alphabet, written from right to left. (The ancient Hebrew alphabet is the same as the Phoenician alphabet, since the ancient Hebrew language is very similar to the Phoenician language.) So, originally, Greek was written from right to left, but very soon it switched from left to right. You can find many ancient Greek vases where both directions are used.

(Note the ancient letters: B stands for E or H, M for Σ). Andromeda and Perseus written from right to left, and Ketos from left to light.

Εdit: Inscriptions were written in boustrophedon, where it starts from left-to-right, then in the next line it goes right-to-left, and all of its characters are mirrored. This alternates between lines. Here is an example (Gortys laws inscription from Crete, 5th century BC):

Asb [CC BY-SA 3.0 (Creative Commons - Attribution-ShareAlike 3.0 Unported - CC BY-SA 3.0)]

The Arabic alphabet developed from the Nabatean script, which in turn is descendant of the Phoenician alphabet.

Source; https://www.quora.com/Why-is-Arabic-...-right-to-left

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