Transmission losses in DC are higher, so you need a lot more wire. You actually can get off the shelf low voltage lighting, though, but it tends to be high end designer stuff.
For the same voltage and wire size, transmission losses in DC are the same or lower (due to skin effect and capacitive/inductive losses). They are only higher when comparing low-voltage DC with higher-voltage AC, but the key difference is the voltage, not DC versus AC.
(As a bonus, DC has lower peak voltage for the same RMS voltage, so the wires need less insulation.)
