because
1) it is baked into the lightmap
2) no support for real lightmaps
3) no dynamic lights
2 is patently false. Lightmaps were originally defined by Quake 1. Newer techniques specifically avoid calling themselves lightmapping for that reason.
And 3 is also patently false with q3map2. You're limited on the number of dynamic lights that can fall on a single surface, and the lights cannot move relative to the surface they're illuminating, but the lighting is not required to be entirely static. Almost nobody bothers to go through the steps to set up dynamic lighting though.
1 is true, but bumpmapping can make a large difference when used properly in Tremulous. For example, compare these two pictures:
Lightmapped w/ Bump and Specularity MapVertex-LitOne is lightmapped with bump and specularity maps used, the other is vertex-lit without either. The vertex-lit image shows I'm not cutting into the geometry to make the details in the lighting, but the bumpmapping is most definately adding shadow and highlight details, in particular in the walls.
Trying to use a bumpmap to add
fine detail to something like diamond-plate flooring though, I agree, is futile. It simply won't work, there's not enough feasable lightmap resolution in a Q3A-based map to accomplish it.