Part of the reason for asymmetric cable modem (DOCSIS) speeds is due to the coaxial medium being essentially half duplex. Operators split the RF spectrum into a

Part of the reason for asymmetric cable modem (DOCSIS) speeds is due to the coaxial medium being essentially half duplex. Operators split the RF spectrum into a lower frequency range for upstream, and a higher frequency range for downstream. Increasing the width of the upstream range would eat into the available downstream bandwidth, and also has other implications in terms of power, noise and overall network design. There is a full duplex version of DOCSIS, but its adoption faces some challenges due to how operators have to re-architect their physical networks to support it. There is also currently a push to extend the downstream spectrum up to higher frequencies, enabling both more downstream bandwidth and also the ability to move the upstream split higher, therefore increasing upstream bandwidth. (Disclaimer: I work with a client who develops products in this space.)