Price. Why spend $4 where $0.05 will do, and will likely never fail anyway? It's a last ditch protection system, not really something that should be tripping all the time.
That said, it used to be fairly common to have breakers on a couple circuits (headlights, for one).
In addition to cost, I'd suppose reliability might be a second consideration. Breakers have contacts in them, which with enough vibration and temperature/humidity cycling might fail, I guess... whereas a fuse has none.
What else.. fuses are faster, better current breaking capacity for DC (at least at this price point). I'd guess the tempco might be better too. (the trip point both devices moves with ambient temperature).