Because the Cobalt SS came with the LSJ or LNF and most builders are using the engine from their donor rather than buying a new engine. Now that ATK doesn't have crate LDKs or LHUs anymore, costs have gone up a lot in the past couple years so you'll see even fewer builds with the new engines.
The LNFs - built with the lost-foam cast block - had an issue with porosity and leaking/seeping coolant. The LDK, which is the European designation for the LNF as Brian pointed out, where built using sand-cast 'Gen 3' blocks, which have strength improvements and no leaks, but got PM rods rather than forged rods like the LNF motors. So building for HP north of 350 would require rods and pistons in the LDK.
I think it was ATK that told me their motor were not exactly LDK. This may just be intake or auxiliary attachments that were different. I think the LDK injectors might be slightly larger so that might be the difference.