Many reasons (very incomplete list): - There is significantly less spectrum available for upstream channel use than downstream channel use. (Something like 37

Many reasons (very incomplete list): - There is significantly less spectrum available for upstream channel use than downstream channel use. (Something like 37 MHz vs. 500-800 MHz.) This means cable operators have to be very frugal with upstream channel allocations and can't reasonably bond together enough upstream channels w/ DOCSIS 3.0 to give everyone symmetric up/down speeds. - Upstream channels are a shared medium using TDMA (time division), which introduces overhead to prevent multiple transmitters (cable modems) from transmitting on top of one another. However, even if your cable modem was the only device on the network, it wouldn't get all of the available upstream timeslots for sending data. - Upstream channels use a less spectrally efficient modulation scheme than downstream channels in order to keep bit error rates low. (The upstream channels are noisier than the downstream channels.) Given identical channel widths, worse spectral efficiency means fewer bits per symbol means less data sent per time unit.