why no modder have done new terrain graphics?

and how about a total conversion mods?

Sure they have. See, e.g., Red Planet. There are some terrain retextures here. If you mean why aren't there new kinds of terrain and features, then that's because there are some serious limitations on the moddability thereof.

The Ea Fantasy Mod Project also modifies the terrain appearance, and would certainly be classified as a total conversion mod. Probably the biggest thing that had been holding back total conversions was the difficulty in creating new units. That's been resolved in the last year to some extent.

We used Loderunner's terrain textures (plus blight which is my own creation with help from others).