Repeating an economic process is made simpler, if your inputs, your process, and your outputs are as similar as possible. In development, none are easy--no two parcels are ever the same, you'll never face the same regulatory environment twice, and the market demand may collapse by the time you get done building it.
Why Mixed-Use Development (MXD) is so hard to do: Low volume. For a developer, it's not worth the time and the trouble to develop the expertise needed for a type of development that you are only going to develop once or twice. Developers conceptualize MXD differently than planners. To planners, it is how all new development should be. To a developer, it's a confusing set of new requirements to be negotiated with the city. When volumes for an economic activity are low, the activity tends to become centralized into a few places where the necessary expertise can be gathered. For MXD, this is more difficult. There are wildly different regulatory and development requirements from jurisdiction to jurisdiction and state to state.