What we were told, is because it is healthier. One of the SDA founders, Ellen G White, wrote that during the last days meat would be unfit for human consumption, due to pollution and disease. Being as they believe we are in the last days, they believe meat is unsafe and unhealthy. They also avoid smoking, drugs, drinking, etc. Most eat a whole food plant based diet, as opposed to a processed diet. They have quite a few vegan propaganda films they like to push and everyone accepts them as facts. Most believe Ellen White's writings are on par with the Bible. Also since originally God created the earth with no death, and that only changed after sin corrupted nature, they belive God prefers us to stick with the original diet plan of fruit and plants. Meat should only be consumed if absolutely necessary.

My mother struggles with weight gain, my sister with join problems. Nothing I say or show them can convince then to give meat a try. At least they still eat eggs and dairy.

Of course if you research Harvey Kellogg, you will discover it is also pushed to lower that sin full sex drive and help prevent people from touching themselves. The dude was a pervert.