G'day Mate
There are more than one way to build a bridge, in this case that bridge is build using Rapid Bridge Replacement (RBR) method, or what they called Accelerated Bridge Construction in the US. Which mean the whole bridge, or a section of a bridge is delivered as a whole to replace the existing structure, it is not possible to have the central tower remain in place or else it would be blocking the automatic loader which tries to deliver the bridge on site.
What you are referring to is the traditional building over pier method, to which you build both ends of the bridge first, then the pier, then the bridge itself and finally run the cable, that method is not generally use today and usually reserved for long/sectioned bridge, for a pedestrian bridge, it is usually preferred the RBR method, because that way you could have less obstruction to traffic flow and the bridge would be finished quicker.
As much as I understand, there is a column in place to hold both end of the bridge, either the column or the bridge failed or the bridge is not secured to the column, it collapsed before the central tower and the cable can be put in place.
Regards
Davos