User blog comment:PerhapsTheOtherOne/Speculative Spitballing: The General Public/@comment-24891101-20150719234250/@comment-82.46.35.19-20150720023235

To Trumpet: The page indicates that the survivors may have wished to keep up the project, given the resources and the potential for some real expansion, regardless of the fact that success looked increasingly slim (this does happen in reality, after all) and I imagine many people did go back to Vale but many would also stay thanks to promises of security (granted this is a slightly stretched interpretation). However, if the Grim struck fast enough, the Valean government probably decided to sacrifice the settlement to give them time to seal the tunnels, condemning the community.

Though I will say that building an urban area right over a condemned tunnel is a bit odd but you can write that off to a) wanting to have an outbreak in a civilian area, rather than....somewhere else and b) the tunnel was originally likely used as the rail line to the underground and emerged in the middle of Vale, abandoning the whole area isn't really a possibility, so they just sealed and built over it. Besides, the tunnel looks like it was largely impenetrable by Grimm thanks to said blast doors and concrete (interesting if we could get a tunnelling species of Grimm actually).

In all things, I would avoid taking the show too seriously when looking at explanations.