Think of the old thin net stuff. You've got a single conductor in that coax cable. Full duplex isn't possible because only one transmitter can be active at any one time. Conversely, with optics you nearly always have a pair of fibers. So it is by nature a full duplex medium.
Yes, arguments could be made against what I just said (you could implement frequency division multiplexing on the coax or something crazy). But where ethernet is concerned, you won't see full duplex via coax. Nor are you likely to see half duplex fiber interfaces (there were fiber hubs for a short period of time but I doubt any are still in use).
