From my experience, there are 'brothers' who are really sincere in trying to help others, but can't tell they are the majority among MS or elders. For them, like pale.emperor mentioned it takes way longer to transfer to the 'next level' - becoming an elder, but it happens nevertheless.
Other reasons seems to be quite popular, as well. But the most important is the jw land system encouraging that. I'd name it under D) peer pressure