Its because of the large span between the two supports. If you take out one support, particularly with force, the bridge immediately either side of it will collapse. That's most of the bridge gone straight away. Then the support the other side of the large span is compromised as the bridge is what is counterbalancing it. As soon as the bridge collapses to one side of the remaining support collumn, all the gravitational force is on the other (like someone suddenly jumping off a see saw).
You can add additional support either side of the large opening. e.g.
But I am guessing the practicality of that all depends on the bridge design, purpose, landscape either side, etc. Quite an easy solution with something like the above where you still have half a mile of bridge either side of the main arches and middle section of the river. This is also 15-20 years newer than the Baltimore bridge I think.