According to Wikipedia many games have been using so far voxel engines for modelling landscape. Why is it only used for landscape and not characters etc?
First of all, it was used for characters sometimes (mostly the cube based version). Just look at the majority of voxel based games (Cube World, Trove, etc.).
Voxels are so powerful, because they allow for complex 3d shapes with overhangs and holes and it makes the model destructable. You usually don't need these options for a character model. It's also very hard to render.