A good manager can manage anything right? Take for example Jack Welch (i.e. former CEO of General Electric), who is universally renown for his management skills. Can you conceive of a business or government project that Jack would not manage effectively? I can. Jack is a damn good manager, but he probably would have done a poor job of managing The Manhattan Project (responsible for building the first atomic bomb). Why? Jack is a world-class executive but not a world-class theoretical physicist. He would have had a hell of a time trying to discern a great atomic bomb design from a disastrous one. And even if this feat were remotely possible, we certainly would have lost the war by the time Jack made a reasonable determination.
If you are going to manage, let alone lead, any non-trivial software development project (and most trivial ones for that matter) you better be an extremely knowledgeable and respected member of the team. Otherwise the geeks are going to chew you up and spit you out. Design sessions will turn into popularity or techno-pissing contests, neither of which is likely to produce great results.
No, Jack Welch, being a world-class chief executive, would have recognized immediately that he was not the right man for the job, and would have, in short order, gone out and hired the best available talent that money could buy. It seems obvious in this example, but over and over again I have seen individuals put into positions of management or leadership, where they have, at best, a rudimentary understanding of the underlying technologies and/or domain. As if charm, good looks, or social graces were enough to get the job done.
This is not to say that being a technically competent individual with good managerial skills is enough either, since I have already expounded at length on the importance of leadership. But if you have these skills, at least you are qualified to be in the game!