A、Because people have to spend too much time and energy to produce it. B、Because too much grain protein is needed to produce it.

In the year 2000, the world is going to have a population of about 8 billion.  Most scientists agree that the most severe problem is food supply.
    Who is going to feed all those people? Where is the food going to come from? Are you going to have enough food? Are we going to produce more artificial food? One way of improving the situation is for people to eat less meat. Why? Because it takes four kilo of grain protein to produce half a kilo of meat protein. Clearly, there is not going to be sufficient meat protein for 8 billion people. Therefore, it will also be necessary to change eating habits because meat is the main part of many people’s food today.
    A possible solution to this latter problem is the soybean. The soybean plant produces beans which have a very high fat and protein content. Scientists can now make these look and taste like real meat. They can also make many other artificial products such as soybean milk, for example, which has a taste of milk and can be used in cooking in very much the same ways as cow’s milk. In fact, one woman in the United States fed her family only on soybeans for a year! She gave them soybean beef, soybean chicken, soybean milk, and sometimes just soybeans. Possibly, we are all going to eat soybeans in file future and finally give up meat completely from our tables.