It's pretty simple really. The runway has been deteriorating for quite some time, they finally did the survey about a year and a half ago from memory but only when Virgin and the others started to threaten to pull out. Survey got done, there was a loan available from the World Bank and also I believe from Japan as well whom were also set to redo their Wharf, but it all fell apart as usual at the last minute due to some minister or another who decided they weren't getting enough money out of it.
There was also talk of constructing an entirely new Airport on Efate, the main island, this muddied the waters somewhat, once again that all came undone rather quickly. The group who were going to do it thought they'd be able to throw money at the Ni-Vans who owned the land they wanted and it'd all be good, these Ni-Vans though had a bit more pride and weren't willing to give up the only real assets and their ancestral lands for money, especially from the arrogant bunch who were offering it, nice to see in a way actually!
Vanuatu also has a lot of corruption within its Government, they recently sent to jail 11+ MPs for corruption, though I think they're all out again now but banned from any positions of power for at least a decade, for what little good that will do.
I was there flying around a couple of weeks after Cyclone Pam hit and there weren't any particular pot holes or large sections breaking up but you could see it starting and who knows what the survey showed underneath as well. They could have had it all done by now but they continually let pride get in the way and as we all know, Pride comes before the fall!