Part of the issue is that blue tarps are made only for temporary use and will fail long before better designs. TarpaFlex makes most of the economy tarps you see in the U.S., and here's what their website says:

"Blue tarps are the tarps that most people come into contact with the most, and they are excellent for common use because they are so very inexpensive. Blue tarps cost just a mere fraction of the price that the heavy duty white tarps may cost, but that is because they are not meant to last a long period of time. In most cases, blue tarps are used just a few times and are easily replaced with just a few dollars. The downside to having traditional blue tarps rather than heavy duty white tarps is that they absorb sunlight and weaken; they easily tear and are not good for transporting or covering and pulling extremely heavy, pointy, or otherwise difficult objects."

Blue tarps are proving to be insufficient in the ongoing relief efforts in Haiti. Bazillions of them were sent after the earthquake and have been put to much harder use than they were designed to handle. The blue tarps are disintegrating under the tropical Haitian sun and are holding up poorly as the rainy season hits.