That's because you probably haven't specified the group within the mesh.
If you actually are trying to make mapmodels within Sauerbraten and export them back as mapmodels (for certain efficient reasons), Since justice edition, there isn't only an .obj-file written when using the command writeojb [name_of_model] but also a .mtl-file. This file stores all data about the settings from groups within the mesh. Deleting it so will obviously result in a mesh which has no skin specified automatically. I've once made a Guide about this but it's probably outdated.
If you've made a model with another programs as Sauerbraten, I would advice you to still do the very same as I would advice you when you made so by using Sauerbraten:
objskin * base.jpg.
Some more details about commands for mapmodels can be found on the node called Mapmodel Prefab.