I imagine the water trucks were ineffective after the PVC caught fire. But the video I saw before the PVC caught fire looked like they should have been able to put it out. I guess its easy to be a back seat driver. I'm sure Marc @ranier1315 is rolling his eyes!
Watching the fire on TV, an official from APD opined that they didn't use foam at first because they didn't know what was burning underneath the bridge. Apparently foam pollutes local water sources so there is some hesitation to use it when water seems to be working.It appears that the conduit was elsewhere some of the time. Google Streetview from a few years ago shows that the underpass was empty.