The lower-voltage hybrids will give you better efficiency...

The lower-voltage hybrids will give you better efficiency than you'd get with a conventional vehicle, but they still don't provide the power you need to do full hybridization. For that, you've got to jump to something higher at least 200V. Given the spectrum of voltages available in the electrical system, you get all the efficiency from the higher voltages, so why spend your time on the 'in-between' voltages?