German regulators instituted the 8 KG carry on at some point of time. Their math was that the latch of the bin is only rated for x KG and with y bags fitting in each bin the limit should be x/y=8Kg. The weight limit is rarely enforced, but what they mind is the size. So if you are travelling like the cliche American with a XXL sized carryon they will flag you during boarding and take a look at size and weight.