It depends on the situation. I'm not familiar with the Site and its various requirements but there would be some reasoning for this - i.e space, the construction of the arches, traffic accommodation or some other practical constraint.You seem to know your stuff.
I was wondering why they don't just build next door to the old bridge and then shift the road there? Wouldn't that be a lot easier/cheaper than moving the bridge?
Bridge's and especially large bridges like this have a ton (no pun intended) of thought provided into them before even the design is undertaken (let alone construction, although the two are linked). I haven't read the report governing the bridge nor am I familiar with the authority (i.e SANRAL or Western Province Department of Transport) behind the bridge.
Road realignment could be a feasible alternative, but I'm not familiar with the project to speculate - roads are generally a lot cheaper than bridges to construct for a given length