Two reasons: Vagueness and Conflict
Vagueness - More and more in publications, instead of making hard fast rules, WTBS use vague possibilities. So instead of being clear, they oft use phrases as "It may be best to..." or "We suggest..." This is bad because the brothers are used to having clear guidelines (which is also not good, but it's the cult-like training). So those brothers will turn those statements into rules to abide by. Later, whatever negatives result, the society is free from liability, as "it was simply a suggestion". But if they don't follow the suggestions, they get reprimanded (whether it works out or not). So the elders are literally grabbed by the chalupas. Thus headquarters are always protected from fault and thus will always push the blame to the locals.
Conflict - Anyone who read Barb Andersons story knows this one well. The WT is under supervision of the Writing Committee. The Circuit Overseer is under the direction of the Service Committee. The committees don't always see eye to eye. In fact, they're in competition at times. Therefore, elders will get two sets of directions on the same issue at times. It forces elders to either wait weeks for a possible solution, or force them to make a position on urgent matters (the ones they need the most help with). And once again, any negatives will always come back to them and never the society.
All the while, the friends suffer....
To make matter worse, it reinforces in the minds of those at headquarters why they should be in control and what happens when the friends "take matters into their own hands". Sadly, you have the local elders who agree. So the cult mentality grows even stronger by both parties. In turn, the congregation is worse.
Then you have the good elders, who are the most powerless. They must either go along, or make a stand. And making a stand, yea, well, ask the elders here.... But others good elders try so hard but once again, because of "mother" being vague or conflicting decisions, they don't stand a chance.
All the while, the elder along with the friends, suffer....
My experience shows most elders suck, but the a great few who are genuine and good are repeatedly told by the Society, "Get the lube ready!"