While I don't disagree with Manishearth; I always thought of it like the old saying:

> "Those who can't do - teach." (1)

But amend it with

> "Those who can't teach - write the text book."

Most of potion recipes were passed down through the ages; there might have been reluctance to alter them. 

It is also entirely possible that the authors of the books were not as gifted in potion-making as Snape was (e.g., to crush the bean with the flat side of a silver blade instead of cutting), and did not think outside the box.

When it would be time to release a new version of potion-making recipes they would add new ones, but leave the old recipes alone; thinking "If it isn't broke - don't fix it"



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(1) Not to say that the teachers at Hogwarts were not exceptional.