This is a good question. Stakeholder first became a term in 1963 in a internal memorandum in the Standford Research Institute. Stakeholders were defined as any group without whose support the corporation could not exist. Since then the idea of the stakeholder was expanded to include, stockholders, employees, suppliers, customers and the like - basically anyone who affected the welfare of the company.
Within this broad definition of what a stakeholder is, there are degrees of power. In other words, some stakeholders have more power than others. So, it stands to reason that those who have the most power have the greatest say in the matter of governance of a company, even in the area of HR strategy.
Also since we are dealing with HR, those who are directly affected by HR will also have a large say. These people would be employees, especially if the employees are unified by a union.