I'm with ewood on this, apart from: I've been lucky enough to often end up in positions where making good progress on the top priorities somehow excuses little to no progress on the low priorities. I'm amazed it's worked out as well as it has but it would appear there may be tiny pockets of sanity in this world.
Are you in upper management, Solearis? Because in my experience, middle management's inability (note spelling) to prioritise stems from upper management's lack of management ability and their frequent inability to keep their eyes on the bigger picture. To some degree, priority is in the eye of the beholder. Middle management often make poor prioritisation decisions due to lack of information (upper management's fault), poor instructions (upper management's fault), lack of training (upper management's fault) etc.
Blaming middle management for the results you list seems to be an ignorance of upper management's responsibilities.
Sadly, the unability to prioritize is a general trait among middle management in my experience. The end result is that everything has the highest priority and those who do not see the big picture will prioritize based on their limited knowledge of the project. And that is the point in time where projects start on the downward spiral of doom.