It has absolutely nothing to do with the lines, but the headends. Coax is very capable of transmitting lots of Data fast. Due to the tree topology of cable however, the headends have to be extremely fast. If everyone on the tree of 100 has 1000 Mbit, that headend needs to have 100 Gbit of capacity. Most of those headends however cap out at 10 Gbit and sometimes service up to 300-500 ports. German cable providers cheaped out and didn't upgrade their infrastructure for quite a while. The coax line technology didn't change in the last 30 years.