In a few religions, e.g., in Hinduism, cattle are considered as sacred animals.
(Apparently, in ancient time, e.g., in Greece or Rome, people had similar beliefs. I guess, cattle were so important animals, useful in many works, thus killing it didn’t make any sense.)
Hinduism is the main religion in India, so cows are sacred there and people don’t eat beef. You can also find in India many cows walking everywhere around (like in today’s picture).