I'm pretty sure someone has posted this recently. Anyway, I may be wrong, but I read that it was something to do with some "weight distribution", since the freewheel is at the BB, meaning centre of the bike, the weight is more centred/neutral. I also think that having the freewheel at the BB means you could engage your chain instantly, reaction time faster. Having it at the hub means you have to engage your chain first (the sag), then engage the hub.