What I'd like to know is, seeing as how there was a good while for folks to get ready for the ministry to unfreeze and finally smite Vivec City like it wanted to for all those years,
why didn't they just break it up before then? The temple had no problem burrowing into the dang thing and clearing out large areas and tunnels, so why not just mine the whole thing out? Even if they could only hollow it out and not break it up completily due to some strange magibabble quirk of Vivec's original enchantment, removing so much of its mass means that even when it does finally get back into gear it might just very well disintegrate on impact.
It was his purpose to have The Ministry frozen in that manner - Consult the 36 Lessons of Vivec and in particular, Sermon Thirty-Three:
When Nerevar returned, he saw the frozen comet above his lord's city. He asked whether or not Vivec wanted it removed.
'I would have done so myself if I wanted, silly Hortator. I shall keep it there with its last intention intact, so that if the love of the people of this city for me ever disappear, so shall the power that holds back their destruction.'
In other words, because he was an arrogent dike.