Actually, vegetarianism is biblical (Genesis 1:29) but later the Bible says we can eat anything that's living. Hmmm....
Some of the Fathers have written that as a result of the fall, it is extremely difficult for us to exist without the help of our fellow creatures, who give of their lives and substance to sustain us. It is simply difficult to maintain high levels of physical activity that the fallen world requires of us ("sweat of the brow" as per Genesis, etc...) without animal products.
It is clear that the Church has never recommended, let alone required, a complete abstinence from meat outside of the context of spiritual struggle -- i.e. monasticism. I think that it isn't uncommon, however, for Orthodox Christians, knowing from living as vegans 1/3 of the year that one can function without meat for extended periods of time, to carry this out further and do without meat much of the rest of the time as well.
It is in part because of this that the forbidding of fasting during certain seasons arose to mandate moderation in all things.