The reason for 12V Halogen is historical. When halogens first came out, they were only producing low voltage ones at any reasonable price. MR16 and those little peanut bulbs were around but you couldn't get higher voltage ones. So, lots of companies made track lights to use the MR16 bulbs because Halogen lamps were so much better than regular tungsten, with better colour, and more light for given watts. So, even with the damn transformer they were great lights, especially for store displays and highlighting things. Anyway, later on they figured out how to make higher voltage versions at reasonable cost and so you could buy PAR lights to fit larger flood fixtures but even today these Halogen PAR lamps are relatively expensive. Finally in the last few years we've seen GU10 halogen bulbs come along, which ironically are the same shape and size as the old MR16, but they take 120V directly. Now, when we shop down at the Home Depot store, most of the track lights use GU10 bulbs and no more transformer.
The transformer approach isn't so horrible since the majority of the "transformer" types don't use big magnetic transformers, they use switching type voltage converters which are more efficient and smaller. So the losses are not too bad (its all relative I guess). Transformer types suffer more by being relatively clunky looking compared to the latest 120V heads. Oh, and transformer types need special dimmers if you want to dim them, whereas the 120V GU10 bulbs do not.