Nearly all cable networks use a low split, cutting off around 42Mhz for upstream bandwidth. Not all bandwidth from 0 to 42Mhz is usable due to external interference, most cable systems are able to get 4 full sized 8Mhz Docsis 3.0 channels into this space and one partial channel of 3 to 4Mhz.
Certain ISPs like Cox have started using OFDMA (Docsis 3.1) upstream channels as it is 50% more efficient than classic Docsis channels and you can operate it closer to spectrum with interference since it can run subchannels at lower modulation
Then there's DOCSIS 3.1, which actually supports up to 1 Gbit/s up, but Comcast still only gives you 35 Mbps on their gigabit plan.
IMO it just comes down to Comcast and other cable providers being cheap and not investing in their infrastructure to provide better upload speeds, even though the tech itself is capable of it.