Why did PMI separated the stakeholders as a separate area in PM-book fifth edition?

My short answer for that is that the PMBOK is not the holy book, and the PMI/PMP thing is not cast in concrete.

This thing (like many other things) evolves and improves over time, and these changes are necessary to push the improvement process.

Regarding the "STAKEHOLDERS" , I remember I have flagged this thing out since PMBOK3rd edition that STAKEHOLDERS need to be separated and dealt with on exceptional basis for2 reasons

1. The stakeholders, stakeholder requirement analysis and the power stakeholders have in deciding the project is a success or failre ... deserves giving them more attention
2. The stakeholders are external entitie (compared to the project team) and they are more concerned about the outcomea and deliverables of the project  ... not how the project management is done, and not how the project is progressing.  Hence, it is better to deal with"stakeholders" as customers and try to satisfy their needs/expectations.

Hopefully this answer sheds some light on the question !!