It's not related to either AC or DC power. The drives themselves require DC regulated power. Some external drives use an AC adaptor, some DC. Inside the external usb enclosure are electronics that convert AC power to DC.
While it's true that 3.5" drives require a 12V line and USB supplies only 5V, this is not the main limiting factor. It's possible to boost 5V to 12V using a boost regulator circuit.
No, the problem is that the USB port cannot supply sufficient current to start spinning the larger drives and maintain that. And even with a 100% efficient 12V boost circuit it can't supply sufficient power to start the drive spinning and maintain it. It's obvious that a 2.5" disk being smaller will not require as much power to spin the smaller platters.
USB can only provide 500mA or 2.5W per port. Which is why you see 2.5" drives with 2 USB connectors. That can draw up to 5W. It may be possible to see a 2.5" drive work with 1 connector. That would mean that the USB port is supplying enough current. Not all computers USB ports are in spec and some may give slightly more than the 500mA needed.
Some drives like the new 3TB WD Cavier Green drives claim to have very low power consumption. However, it's near 13W in total to make it spin up. Most of that is needed on the 12V rail with only 2W on the 5V rail. While idle it'll use around 3-4W but when busy near 7W of power. Well beyond what USB can supply even with 2 connectors.
With 4 USB connectors we could in theory supply 10W of power. But, it's still not enough for the drive start up.
Now do you get the picture? it's just simpler to have an external power supply. Although I see that the eSata standard will soon supply drive power too as well as greater speed so I guess all these problems will go away.