There's a bridge near my house whose westbound lane has been blocked off for ages. It hasn't been usable for at least two years. I'm not even sure what's wrong with it, just that it's blocked off and I've never seen any kind of work being done on it at all. The town installed a traffic light on the remaining lane of the bridge rather than work on fixing it.
As has been pointed out, bridges can be expensive. If existing traffic flows aren't sufficient to justify building a replacement, they may well be doing the smart thing in making optimum use of the one lane while it's still available.
