I have heard of the following reasons

• They pray to Guan Yin (Goddess of Mercy) and the father Guan Yin was the "king of cows" and thus, it is a respect we show to deity Guan Yin

• Buddhists respect cows for their contribution towards mankind, as they supply milk to man, and do all the hard work in the field

• Gautama Buddha, the founder of Buddhism was born a Hindu, and thus abstinence from beef is a Hindu tradition that naturally goes to China hand-in-hand with Buddhism

However, based on my knowledge, beef is widely eaten in countries like Japan, Korea, China, Taiwan, Hong Kong, and Vietnam. Also, I would like to know if there's anything to do with the difference between Theravada and Mahayana branches. Which might explain beef's prevalence in the above mentioned countries.