The more complicated the modulation is, the more processing power is required. DSL modems use very complex modulation to push their data across the wire and bur

The more complicated the modulation is, the more processing power is required. DSL modems use very complex modulation to push their data across the wire and burn a few Watts doing so (there's also a fairly powerful line driver, but that's not the main power consumer, the DSP/modem itself is). Of course, SNR is another big one. You have to actually be able to distinguish the voltage levels within a symbol period from each other. Another thing: PCIe is meant to be low-latency, which just rules out a lot of the modulation and error-correction techniques that are traditionally used to enhance channel utilization, like having multiple levels of FEC and then interleaving FEC symbols temporally. That's done basically always where latency allows, e.g. in storage media, digital TV and so on.