Why is resource leveling in a multi-project environment not successful?

Interesting question. In a multi project environment people work simultaneously in several projects.  Resource allocation is the biggest challenge in such an environment. Resource leveling over multi projects demands priority rules between projects. Planners have to define these priority rules. After leveling the planner has a hard time to understand the leveling rules of the software system. He does not recognize the end result.
 
Secondly, resource leveling assumes a deterministic world. I can assure you that determinism does not exist in a multi project environment. Why? To much variations in task completion and to many interdependencies between projects because of the shared resources. Every time you level, reality will hit you soon. 

Thirdly, for 30 years the academic community tries to develop heuristic methods for optimizing the resource allocation. I know some of the professors in this world who decided to stop this avenue because it does not work! In contrast with Advanced Planning & Scheduling in manufacturing environments, this does not work in multi project environments. To much variation.

But there is one way out. Add a lot of safety to every task to deal with the uncertainty. Then do the resource leveling. You will notice that it is much more stable. But don't be sad if you discover that the efficiency of your company dropped down significantly and lead times of your projects expand.

Conclusion: be careful in believing the ambassadeurs of Project Management software that are exited about their resource leveling functionality.  

Their are two alternatives concepts: FLOW MPM and CCPM. Although CCPM is also a type of resource leveling within one project.