I think the reason Blue Shift didn't feature multiplayer is because it wasn't really any different to the original HL gameplay wise.
It was a map pack with some new models. There is nothing in Blue-Shift that couldn't be done with HL1 because the enemies and weapons are all the same.
Opposing Force on the other hand added new weapons and mechanics which made the MP aspect more entertaining. Blue Shift MP would just be HLDM with Barney's hands.