Practicality
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Slow changes and **authority say-so**. An example would be the Japanese system:

 - Kanji system was copied from China.
 - Hiragana is created by aristocrat women to communicate with each other. The system evolves with small changes. There were many variants of a syllable, until authority say-so that there shall be only one.
 - Katakana was created by men and used in official documents, somewhat an authority say-so.

The all this mess has been cleaned to the modern format by multiple authority say-so, **often ratifying the most common practice at the time**. That bold part is really important because the change often goes to multiple directions and the authority say-so is a way to keep it together.

It would be really hard for English to become vertical because the letters have evolved to such that they are best red horizontally. The verticality of Chinese is because the ink and sleeves are not compatible.