If I simply say, "We don't eat meat because we don't eat anything after killing it. " I know many will reply -"Plants and crops are also living." So I think it is better to explain it in detail. I support my statement by two giving out two reasons.
First,Out of Mercy. Causing pain is not justified. All animals, insects, killed by any means feel pain of being killed. They have a nervous system that makes them feel the pain of being killed. Cow, pig, hen, fish, crab all have a nervous system and thus feel the pain. So it's unjust to eat them as a diet.
Plants on the other hand are without nervous system. The various chemicals present in them make them respond to the various physical factors such as light, heat, minerals etc. but they are devoid of nerves and can't feel. So bring them on.
Second reason is- Who you are? If you recall the evolution of life on this planet, that you must have studied in your junior classes - you will find - plants came first, follwed by creepers,carnivores herbivors, and then humans ( I know it's more complicated). If you observe the pattern of diet of the animals as per the evolution you will find, the later animals I mean the animals that evolved later are herbivores.
The complex the digestive system of a ceature is -the lower level of food it takes. Let me explain it a bit. Grass, leaves, fruits are lower levels of energy. We humans, cow, buffalo, deer all have a good developed diegestive system that can extract maximum energy from these lower level energy sources (grass, leaves etc. ) Whereas the lower level animals -don't have a that developed digestive system that can extract/convert the energy present in the lower level foods into their own blood. So they don't have any option but to eat other animal who has capability of eating lower level energy food. That's why carnivores rarely eat carnivores.
We humans have been blessed by the Almighty to accept both types of food. But we should be merciful too. (consider the first reason). So if other options are available then you should go for them rather than taking meat you should go for vegetarian food. Leaving the flesh for the hard days. I think I'm able to drive the point home. Isn't it?
Please: There must be some other reasons too but my micro sized brain at this time is capable of getting these two reasons only. If you have a different and scietifically true reasoning, do put it below in the comments. I would be happy to read it and will also include in the post.
Again I would like to say - the explanation is based on my understanding.