Most of the new managers get promoted from within, and the way we get promoted isn’t necessarily the best. You see, typically, you do a great job in your role, and you get rewarded with a promotion to a manager. My experience stems mostly from sales organizations, but I’m sure it’s the same across the board — the best individual contributors don’t necessarily make the best managers.
And the opposite is true as well — being a manager doesn’t require experience in any of the things your team is doing. Now, most people will frown upon the latter statement because nobody wants to be managed by “what does he know.”