What can be possible reasons not to implement agility effectively in the organization?

You can boil down Agile to increments, iterations and continuous improvement. Applying these concepts in a large organization can be challenging. Doing it from the bottom-up will be difficult as reporting on Agile is quite different. Don't be surprised if senior management doesn't appreciate your burn charts and velocity numbers. Expect them to still ask when it will be done. (In Agile, you're only done of your iteration.)