The deterministic world of working with code and packages and classes is very, very different from the non-deterministic world of working with people working on code (or other non-code business functions).
I think a lot of developers make the mistake that management is a natural extension of being a decent developer. It's a whole other animal. Half the people I know who get into it hate it and back out. You have to completely pervert your perspective, as it's no longer about you or what you get done, it's about everyone else and what they get done. That's a huge leap many don't successfully make, IMO.