The main problem is that if you are minimizing the voltage to the minimum that can be safely distinguished for binary, you must, by necessity, be introducing twice that voltage to introduce another level. You can't just cut your already-minimized voltage in half to introduce another level; you already minimized the voltage for binary.
50 years ago this may not have been such a problem, but now that we care a lot, lot more about the power consumption of our computing, and a lot of that power consumption is based on voltage (and IIRC often super-linearly), so a tech that requires us to introduce additional voltage levels pervasively to our chips is basically disqualified from the very start. You're not sticking one of these in your server farm, phone, or laptop anytime soon.
50 years ago this may not have been such a problem, but now that we care a lot, lot more about the power consumption of our computing, and a lot of that power consumption is based on voltage (and IIRC often super-linearly), so a tech that requires us to introduce additional voltage levels pervasively to our chips is basically disqualified from the very start. You're not sticking one of these in your server farm, phone, or laptop anytime soon.