My guess would be that, like other 80s properties, many of the first generation of fans have now grown into adults with disposable incomes. I know thats the case with me and my circle.
Ten-fifteen years later they've started making big monies compared to a kid, and nostalgia is a powerful marketing tool.
Once the iPad generations take over I suspect Games Workshop will have it tougher.
I never completed it, but since then I have played the smaller (Monopoly board size) game, Warcry.
The friends I discovered were playing the full-size games spend hundreds every week or two.