In scrum there's inherent tension between a product owner and a scrum master. Both are managers of some sort with one managing the product while the other is managing people and the process. I've seen such constellations working really well several times but only if both persons are equally strong. If either a product owner or a scrum master starts to dominate, one of them quickly assumes a classical project manager role while the other just becomes a subordinate of the other. The main problem of a classical project manager is that (s)he is responsible for product, people and process, which is just too much for a single person. Product requires you to focus broadly and outside of your organization, while people and process require you to focus on execution and protecting the team from external influences. Another big problem of a classical project manager is that the tension between the product requirements and the execution happens inside the project manager's head. This tension is therefore never verbalized and communicated with the team, which makes for a omniscient project manager and ignorant team. Consequently, instead of delegating responsibility, the project manager starts delegating tasks, which might work for some time but it never gives good results over the long haul.