A lot of the answer is that the types of structures you can build with concrete that isn't rebar reinforced is fairly limiting. You're pretty much stuck with arch bridges, dams, and domed buildings. Unfortunately, arch bridges don't scale that well (they take concrete proportional to length cubed), and people like buildings in shapes other than domes. As a result, you often have to have designs that hold together partially under tension, and the cost is it won't stay up for 2000 years (if you use rebar which rusts).
That said, in the past few decades there has started to be experiments with fiber reinforced concrete. This has the significant advantage of not corroding, but costs a good bit more than traditional rebar. That said, it might be the future for buildings where long life is desired.
Multiple arches helps in some cases, but gets really expensive if you have to bridge a deep gap (especially over water where de-watering is needed) The massive advantage of a suspension bridge is that you get a really big span without anything in the middle.
That said, in the past few decades there has started to be experiments with fiber reinforced concrete. This has the significant advantage of not corroding, but costs a good bit more than traditional rebar. That said, it might be the future for buildings where long life is desired.