Plant forms are abundantly available in nature. They are easy on the human digestive system and are easily assimilated. It does not involve hunting and shedding blood-there is no conflict with other animals and fellow humans
In the prehistoric days, our ancestors living in the forests, were unable to find the right kind of plants for nutrition. They were accustomed to eating meat when animals died or were eating leftover meat by wild animals. Other times, they fed on aquatic animals washed ashore or when seasonally available. Thus they got their nutrition mainly through meat/fish.
With the development of Stone Age tools, they further improved their skills of hunting.However, when they were not able to get the meat, homo sapiens learnt survival eating fruits and plant food. The simplicity and safety of vegetarian-way became a catalyst to trying out new ways of cooking food and experiment with abundant varieties available in the nature.There was no looking back!
Other advantages are:
Easy availability
Easily cultivated and replenish-able
Wide and huge verities
Tasty even when uncooked
Suitable for all ages
Fibrous
Medicinal
Nutritional
No retaliation from the hunted animals and chances of being killed in the hunting
Easy on environment
Amazing By-products
Easy on the digestive system
Easy assimilation
Some of them are therapeutic
Animal meats have shorter shelf life and difficulty and expensive in storing
From the spiritual and religious perspective, some religions such as Hinduism, Buddhism, Jainism and other similar convictions promoted Vegetarianism as a way to attain Spiritual Nirvana.
Many people have been of late interested to know the benefits of being vegetarian and how it can be adapted if found good to switch from meat eating to vegetarianism. For more information visit websites like https://www.pcrm.org/