Welcome fellow (and future) vegan organic gardeners. I'll begin by saying I've been working on a revolutionary growing system with ganja guru Kyle Kushman. He calls it Kushman's Veganics and it includes all aspects of gardening.
And secondly, I am not a vegan, nor do I promote a vegan diet for humans. I get my meat from the farmer's market, and it is spectacular, support local ranches and eff fast food. Sorry, I digress.
It's all about the medicine. We are literally making medicine for sick people, often people with severely compromised immune systems. So getting rid of the animal sources removes the risk of any weird animal pathogens being consumed. Remember my beautiful canna-friends, sick people often consume the medicine raw as juice.
It is also about reproducing nature. In the wild plants feed off of the soil. Soil is mostly broken down plants and some rocks. Very little of soil is of animal origin. So are you starting to see the idea? Broken down plants=good, rocks=okay in small amounts, animal origin=okay in very small amounts, chemical=no way!