The main reason may be the killing of animals is treated as a sin and one should not kill an animal for eating. This is believed many people and that is the reason they stop eating non vegetarian.
However, this is a new information for me and I have mentally noted it.
Beware! I question everything and everybody.
The original religion was different when formulated by its founders but with time it evolved to accommodate the notes of dissent among its people and in that process got subtle changes in its working principles.
Some historians term it as abberation in religions but the people belonging to that religion call it the allowed deviations or permitted negotiations.
Unfortunately, the gullible and simple people take it as the God's word and follow these changes.
In North also, many people do not take onions, garlic etc on Tuesday which is a fast day for many worshipping Hanuman.
Strictly speaking if something is forbidden in a religion, it should not be consumed but as I mentioned earlier these are the modified or aberrated forms or evolution of the religion with time.
Knowledge is power.
" It is better to be hated for what you are than to be loved for what you are not" ... Andre Gide