DOCSIS 3.1 is capable of going up to 4096QAM. That doesn't mean providers will actually go that high, as denser constellations require higher signal to noise ratios to function properly. To visualize how this works, say you have a 4"x4" piece of paper. Divide it up with lines every inch, giving you an 8x8 grid. This is equivalent to 64QAM. Now, take that same size paper and divide it up with lines every half inch, giving you a 16x16 grid. This is equivalent to QAM256. Then, take that same size paper and divide it up with lines every 1/8th of an inch, giving you a 64x64 grid. That's 4096QAM. Each one of those squares represents a specific value. In 64QAM, it gives you a 6 bit value, since 2^6=64. In 256QAM, 8 bits, since 2^8=256. In 4096QAM, 12 bits, since 2^12=4096. Now imagine that you're throwing a dart at the grid, and which square you hit in there gives you the specific value. 64QAM gives you much more room for error in where you hit to still get the number you intended to hit, while you have to be incredibly precise on 4096QAM because your squares are 1/8"x1/8". The more precise you can be (the cleaner your signal is), the more bits you can represent with each hit. Do it over 5 million times per second, and that's a basic conceptual description of what digital cable modulation looks like. There's obviously a lot more that's going on, but this should be enough to make the point. Cable providers have to balance performance of the system against the requirements for customers to be able to receive the signals, and 256 is a sweet spot.
HDHomeRun PRIME handles HEVC fine. The problem is that Comcast doesn't have 4K channels because they don't have enough bandwidth for traditional 4K channels, and that's unlikely to change. What 4K they do have is delivered as VOD via internet bandwidth rather than TV bandwidth, and that can only be accessed by their devices, not by CableCARD devices.