Objective-C had the primary benefit (over Smalltalk) of being a superset of C, which permitted immediate and easy interaction between Objective-C and C source code. It also integrated better (which was critical, Smalltalk doesn't force using the image model for source code, but it was typical) into existing development workflows. Those are the kinds of practical concerns that can't be neglected when considering language adoption, especially since NeXT (main early adopter) was building on the lineage of the Unix-family of operating systems.