Beavers and cubs attracts the kids who's parents have decided to send them. So it's a mixture of all sorts - academic kids, sporty kids, outdoorsy kids, we have them all. Young kids also tend to do quite a few clubs, so they can fit it beavers/cubs, while also doing swimming, football, chess etc.
Once they get to late cubs and scout age (around 10-12) then some start to drift away, and I would say that on average, it's the quirkier kids that are more likely to stay. At this age, homework increases, plus sports clubs tend to demand more time (for those who are quite sporty, they may have two or three sports meetings a week). It's also not as cool to be a scout as it is to be a footballer!
It also seems to be, on average, that the more academic kids stay on.
My two football loving boys left at the end of cubs, but my DD, who is not particularly sporty, but is quite quirky and academic, has stayed all the way from beavers to explorers, and is also a young leader. Thinking about it, I have three young leaders who help at cubs, and they are all none sporty, all go to super-selective grammar schools, so it certainly appeals more to a certain type as they get older.