Originally Posted by MarkZ
You're feeding off a 12v source no matter what, that's correct. But amplifiers convert that 12v into a higher voltage in order to achieve greater amounts of power. This requires a more complex and sizable power supply in the amp, which tends to take up about 1/4 of the total real estate in a typical amplifier (and about half of the heatsink). So it's not exactly trivial to stick a power supply that does that into something small and confined like a head unit, which is why most head units out there were faced with that 12v limit.
ETA: This, of course, compared to home application where you're working with voltages sufficiently high to make this sort of thing moot.