Managers are often promoted based on their technical acumen. The big challenge occurs when they are forced to step back from individual execution and lead a multi-generational team without adequate preparation.
If being a great manager was easy, there would not be stacks of management books in every bookstore in North America. Do a simple search on your computer and you will see dozens of titles, each purporting to be the essential expert on the topic.
In most organizations, employees are promoted based on technical acumen: They are spectacular at the role they were in, and now they will manage others in that capacity.