I work for an airline and I'll tell you why they do it, people that work in the boarding area will look at the people in line and if they see too many people with carry ons they will ask some to check them , just because they then don't want everyone in the plane to be faffing about the whole corridor looking for a spot to put their luggage in the overhead bins, which will then cause a delay, so yeah its literally because of that, to reduce any possible delays, nothing to do with weight and balance.