Put simply, maintenance is a local issue. Marohn cites the example of his home state of Minnesota, which has more than 1000 deficient bridges in need of significant rehabilitation or replacement. Federal infrastructure money only comes in large quantities for new construction. If Minnesota passes up on the chance to build a new bridge for $670 (while maintaining all 1,149 deficient bridges would cost $500) that money would simply go to another state.
Therefore, politicians, for whom new construction projects aid re-election campaigns, are incentivized to seek new projects at the expense of future generations who will have to foot the repair bills.