bulbs made to be dimmed. 110V. Using the 110 Volt current. So they can use the same circuit as other bulbs, like the old style incandescent ones we have seen for generations.
That may be the simplest answer to tell you why they haven't made use of DC low volts.
Halogen bulbs that don't use low voltage, use regular house current.
Bulbs like MR16, GU10, and T4 bulbs can take 110 volts. AFAIK.
There is no transformer.
That may be the answer you wanted.
On the other hand, i don't know whether you were asking about a specific dimmer called "Duo ..."
david