Two HUGE reasons:
1. COST. It would be incredibly expensive to expand the monorail. Current figures for expansion run about $90-$100 million per mile. Each train would run $25m. Add stations, maintenance facilities, switchbeams, and the cost skyrockets even more.
2. LOGISTICS. Bottom line is that as much as people would want a monorail, it really would cause more problems than it would solve. Disney has a unique traffic flow pattern, and requires a highly flexible demand responsive system. Although fixed guideway systems might work great in other applications, only buses can provide the flexibility that we need for our specific application.