I bet that their thinking is that if they focus on it being competitive (you dont even get times do you ?) that they will scare off some of their delicate entrants who wouldnt want to race/break sweat.
The last time I went to one I saw three women walking round the course each scoffing a family bar of dairy milk as they waddled.
Do they emphasise that the charity doesnt get anything unless you raise money on top of the entry fee now ? I always thought that was quite well hidden.