Mains is 120v, mr16 gu5.3 are 12v ac, so the fitting includes a transformer to step down. I think the lighting manufacturers don't like 12V fittings because it adds to their cost, but the 12V halogen bulbs do have a nice whiter light than 120V bulbs
If you are buying a new fitting of any type for LED bulbs, please make sure the bulb is properly ventilated otherwise it will fade or fail prematurely.