Any 60 year old organization with a large budget is going to have multiple instances of this happening.
Yes there are a few good reasons, but the population it applies to have largely been selected out by the postdoc phase (where I would say you have had sufficient experience with crushing scientific and engineering failure onself and watching others to be effective) and the pool of candidates therefore is vanishing.
Reasons are myriad.
100% of the PMs I've worked with had tenure and successful labs prior to becoming PMs, and went back to their institution & restarted their labs after leaving DARPA. The reason for leaving your lab to be PM is fairly obvious: controlling the funding gives you a lot of leverage for shaping the priorities of the field, in a way that merely running your own lab doesn't.
Maybe your division sucked. I've only ever worked with highly competent PMs, and all of the programs I've worked on ended in commercialization.
Anyways, prosecuting individual cases doesn't seem like a particularly good way of evaluating the effectiveness of an agency.
That makes sense.