Good question. The answer is: infrastructure. Read this from Dr. Dobb's. In case anyone doesn't know who Dr. Dobb's is, ask any software developer geek. Dr. Dobb's is a world wide software development (SD) authoritarian.
"By 2010, there will be 12.4 million WiMAX subscribers worldwide..."
"Initially, the technology will find its greatest success in developing countries. However, the technology won't really take off widely until mobile WiMAX is launched..."