Projected population growth
Sustainable agriculture aims for both high, maintainable yields and minimal environmental impact. Combining these two factors is difficult, as things that make plants grow aren't always good for the environment. For example, plants need nitrogen fertilizer to grow well; however, plants do not absorb all of the nitrogen in fertilizers and the rest is loaded into waterways, eventually perturbing marine ecosystems.
Tillman, 2002 also discusses the sustainability aspect of meat production (or lack thereof). The following illustrations probably need no further explanation.