12VO would make PSUs cheaper, and there would be fewer, smaller connections from the PSU to the motherboard. The issue is that motherboards become more expensive to produce.
From my limited research on the subject, 12VO isn’t a bad standard. IMO it needs to happen. That, or a similar standard.
EDIT: take a look at this article:
https://overclock3d.net/reviews/power_supply/atx12vo_tested_-_the_future_of_power_supplies/1
12VO simplifies everything. Fir example, since SATA power now comes from the motherboard, in theory you only need 1 cable instead of 2. That isn’t the case for the board in the article, but I can see it happening.