Novalogic released several games using voxels. For example Comanche Gold and Delta Force 2. The game Outcast by Appeal uses voxels for the environment and polygons for animated objects like characters. There are several more games from between 1992 and 1999. So voxels are also there for a long time already I guess the biggest problem for voxels was the increasing resolutions games were rendered with, as you needed more details and thus more and smaller voxels, which in the end turned out to be slower than using polygons.